Top 180+ SQL Interview Questions [2022]- Great Learning

SQL Interview Questions

Are you an aspiring SQL Developer? A career in SQL has seen an upward trend in 2022, and you can be a part of the ever-so-growing community. So, if you are ready to indulge yourself in the pool of knowledge and be prepared for the upcoming SQL interview, then you are at the right place.

We have compiled a comprehensive list of SQL Interview Questions and Answers that will come in handy at the time of need. Once you are prepared with the questions we mentioned in our list, you will be ready to get into numerous SQL-worthy job roles like SQL Developer, Business Analyst, BI Reporting Engineer, Data Scientist, Software Engineer, Database Administrator, Quality Assurance Tester, BI Solution Architect and more.

Great Learning has prepared a list of the top 10 SQL interview questions that will help you during your interview.

Now, without any further ado, let’s kick off our SQL learning journey.

Let’s get started!

Basic SQL Interview Questions

All set to kickstart your career in SQL? Look no further and start your professional career with these SQL interview questions for freshers. We will start with the basics and slowly move towards slightly advanced questions to set the pace. If you are an experienced professional, this section will help you brush up on your SQL skills.

What is SQL?

The acronym SQL stands for Structured Query Language. It is the typical language used for relational database maintenance and a wide range of data processing tasks. The first SQL database was created in 1970. It is a database language used for operations such as database creation, deletion, retrieval, and row modification. It is occasionally pronounced “sequel.” It can also be used to manage structured data, which is made up of variables called entities and relationships between those entities.

What is Database?

A database is a system that helps in collecting, storing and retrieving data. Databases can be complex, and such databases are developed using design and modelling approaches.

What is DBMS?

DBMS stands for Database Management System which is responsible for the creating, updating, and managing of the database. 

What is RDBMS? How is it different from DBMS?

RDBMS stands for Relational Database Management System that stores data in the form of a collection of tables, and relations can be defined between the common fields of these tables.

How to create a table in SQL?

The command to create a table in SQL is extremely simple:

 CREATE TABLE table_name (
	column1 datatype,
	column2 datatype,
	column3 datatype,
   ....
);

We will start off by giving the keywords, CREATE TABLE, and then we will give the name of the table. After that in braces, we will list out all the columns along with their data types.

For example, if we want to create a simple employee table:

CREATE TABLE employee (
	name varchar(25),
	age int,
	gender varchar(25),
   ....
);

How to delete a table in SQL?

There are two ways to delete a table from SQL: DROP and TRUNCATE. The DROP TABLE command is used to completely delete the table from the database. This is the command:

DROP TABLE table_name;

The above command will completely delete all the data present in the table along with the table itself.

But if we want to delete only the data present in the table but not the table itself, then we will use the truncate command:

DROP TABLE table_name ;

How to change a table name in SQL?

This is the command to change a table name in SQL:

ALTER TABLE table_name
RENAME TO new_table_name;

We will start off by giving the keywords ALTER TABLE, then we will follow it up by giving the original name of the table, after that, we will give in the keywords RENAME TO and finally, we will give the new table name.

For example, if we want to change the “employee” table to “employee_information”, this will be the command:

ALTER TABLE employee
RENAME TO employee_information;

How to delete a row in SQL?

We will be using the DELETE query to delete existing rows from the table:

DELETE FROM table_name
WHERE [condition];

We will start off by giving the keywords DELETE FROM, then we will give the name of the table, and after that we will give the WHERE clause and give the condition on the basis of which we would want to delete a row.

For example, from the employee table, if we would like to delete all the rows, where the age of the employee is equal to 25, then this will be the command:

DELETE FROM employee
WHERE [age=25];

How to create a database in SQL?

A database is a repository in SQL, which can comprise multiple tables.

This will be the command to create a database in sql:

CREATE DATABASE database_name.

What is Normalization in SQL?

Normalization is used to decompose a larger, complex table into simple and smaller ones. This helps us in removing all the redundant data.

Generally, in a table, we will have a lot of redundant information which is not required, so it is better to divide this complex table into multiple smaller tables which contain only unique information.

First normal form:

A relation schema is in 1NF, if and only if:

  • All attributes in the relation are atomic(indivisible value)
  • And there are no repeating elements or groups of elements.

Second normal form:

A relation is said to be in 2NF, if and only if:

  • It is in 1st Normal Form.
  • No partial dependency exists between non-key attributes and key attributes.

Third Normal form:

A relation R is said to be in 3NF if and only if:

  • It is in 2NF.
  • No transitive dependency exists between non-key attributes and key attributes through another non-key attribute

What is join in SQL?

Joins are used to combine rows from two or more tables, based on a related column between them.

Types of Joins:

INNER JOIN − Returns rows when there is a match in both tables.

LEFT JOIN − Returns all rows from the left table, even if there are no matches in the right table.

RIGHT JOIN − Returns all rows from the right table, even if there are no matches in the left table.

FULL OUTER JOIN − Returns rows when there is a match in one of the tables.

SELF JOIN − Used to join a table to itself as if the table were two tables, temporarily renaming at least one table in the SQL statement.

CARTESIAN JOIN (CROSS JOIN) − Returns the Cartesian product of the sets of records from the two or more joined tables.

SQL joins

INNER JOIN:

The INNER JOIN creates a new result table by combining column values of two tables (table1 and table2) based upon the join-predicate. The query compares each row of table1 with each row of table2 to find all pairs of rows which satisfy the join-predicate.

SYNTAX:

SELECT table1.col1, table2.col2,…, table1.coln
FROM table1
INNER JOIN table2
ON table1.commonfield = table2.commonfield;

LEFT JOIN:

The LEFT JOIN returns all the values from the left table, plus matched values from the right table or NULL in case of no matching join predicate.

SYNTAX:

SELECT table1.col1, table2.col2,…, table1.coln
FROM table1
LEFT JOIN table2
ON table1.commonfield = table2.commonfield;

RIGHT JOIN:

The RIGHT JOIN returns all the values from the right table, plus matched values from the left table or NULL in case of no matching join predicate.

SYNTAX:

SELECT table1.col1, table2.col2,…, table1.coln
FROM table1
RIGHT JOIN table2
ON table1.commonfield = table2.commonfield;

FULL OUTER JOIN:

The FULL OUTER JOIN combines the results of both left and right outer joins. The joined table will contain all records from both the tables and fill in NULLs for missing matches on either side.

SYNTAX:

SELECT table1.col1, table2.col2,…, table1.coln
FROM table1
Left JOIN table2
ON table1.commonfield = table2.commonfield;
Union
SELECT table1.col1, table2.col2,…, table1.coln
FROM table1
Right JOIN table2
ON table1.commonfield = table2.commonfield;

SELF JOIN:

The SELF JOIN joins a table to itself; temporarily renaming at least one table in the SQL statement.

SYNTAX:

SELECT a.col1, b.col2,..., a.coln
FROM table1 a, table1 b
WHERE a.commonfield = b.commonfield;

How to insert a date in SQL?

If the RDBMS is MYSQL, this is how we can insert date:

"INSERT INTO tablename (col_name, col_date) VALUES ('DATE: Manual Date', '2020-9-10')";

What is Primary Key in SQL?

Primary Key is a constraint in SQL. So, before understanding what exactly is a primary key, let’s understand what exactly is a constraint in SQL. Constraints are the rules enforced on data columns on a table. These are used to limit the type of data that can go into a table. Constraints can either be column level or table level. 

Let’s look at the different types of constraints which are present in SQL:

Constraint Description
NOT NULL Ensures that a column cannot have a NULL value.
DEFAULT Provides a default value for a column when none is specified.
UNIQUE Ensures that all the values in a column are different
PRIMARY Uniquely identifies each row/record in a database table
FOREIGN Uniquely identifies a row/record in any another database table
CHECK The CHECK constraint ensures that all values in a column satisfy certain conditions.
INDEX Used to create and retrieve data from the database very quickly.

You can consider the Primary Key constraint to be a combination of UNIQUE and NOT NULL constraint. This means that if a column is set as a primary key, then this particular column cannot have any null values present in it and also all the values present in this column must be unique.

How do I view tables in SQL?

To view tables in SQL, all you need to do is give this command:

Show tables;

What is PL/SQL?

PL SQL stands for Procedural language constructs for Structured Query Language. PL SQL was introduced by Oracle to overcome the limitations of plain sql. So, pl sql adds in procedural language approach to the plain vanilla sql.

One thing to be noted over here is that pl sql is only for oracle databases. If you don’t have an Oracle database, then you cant work with PL SQL. However, if you wish to learn more about Oracle, you can also take up free oracle courses and enhance your knowledge.

While, with the help of sql, we were able to DDL and DML queries, with the help of PL SQL, we will be able to create functions, triggers and other procedural constructs.

How can I see all tables in SQL?

Different database management systems have different queries to see all the tables.

To see all the tables in MYSQL, we would have to use this query:

show tables;

This is how we can see all tables in ORACLE:

SELECT 
    table_name
FROM
    User_tables;

This is how we can extract all tables in SQL Server:

SELECT 
    *
FROM
    Information_schema.tables;

What is ETL in SQL?

ETL stands for Extract, Transform and Load. It is a three-step process, where we would have to start off by extracting the data from sources. Once we collate the data from different sources, what we have is raw data. This raw data has to be transformed into the tidy format, which will come in the second phase. Finally, we would have to load this tidy data into tools which would help us to find insights.

How to install SQL?

SQL stands for Structured Query Language and it is not something you can install. To implement sql queries, you would need a relational database management system. There are different varieties of relational database management systems such as:

Hence, to implement sql queries, we would need to install any of these Relational Database Management Systems.

What is the update command in SQL?

The update command comes under the DML(Data Manipulation Langauge) part of sql and is used to update the existing data in the table.

UPDATE employees
SET last_name=‘Cohen’
WHERE employee_id=101;

With this update command, I am changing the last name of the employee.

How to rename column name in SQL Server?

Rename column in SQL: When it comes to SQL Server, it is not possible to rename the column with the help of ALTER TABLE command, we would have to use sp_rename.

What are the types of SQL Queries?

We have four types of SQL Queries:

  • DDL (Data Definition Language): the creation of objects
  • DML (Data Manipulation Language): manipulation of data
  • DCL (Data Control Language): assignment and removal of permissions
  • TCL (Transaction Control Language): saving and restoring changes to a database

Let’s look at the different commands under DDL:

Command Description
CREATE Create objects in the database
ALTER Alters the structure of the database object
DROP Delete objects from the database
TRUNCATE Remove all records from a table permanently
COMMENT Add comments to the data dictionary
RENAME Rename an object

Write a Query to display the number of employees working in each region? 

SELECT region, COUNT(gender) FROM employee GROUP BY region;

What are Nested Triggers?

Triggers may implement DML by using INSERT, UPDATE, and DELETE statements. These triggers that contain DML and find other triggers for data modification are called Nested Triggers.

Write SQL query to fetch employee names having a salary greater than or equal to 20000 and less than or equal 10000.

By using BETWEEN in the where clause, we can retrieve the Employee Ids of employees with salary >= 20000 and <=10000.

SELECT FullName FROM EmployeeDetails WHERE EmpId IN (SELECT EmpId FROM EmployeeSalary WHERE Salary BETWEEN 5000 AND 10000)

Given a table Employee having columns empName and empId, what will be the result of the SQL query below? select empName from Employee order by 2 asc;

“Order by 2” is valid when there are at least 2 columns used in SELECT statement. Here this query will throw error because only one column is used in the SELECT statement. 

What is OLTP?

OLTP stands for Online Transaction Processing. And is a class of software applications capable of supporting transaction-oriented programs. An essential attribute of an OLTP system is its ability to maintain concurrency. 

What is Data Integrity?

Data Integrity is the assurance of accuracy and consistency of data over its entire life-cycle, and is a critical aspect to the design, implementation and usage of any system which stores, processes, or retrieves data. It also defines integrity constraints to enforce business rules on the data when it is entered into an application or a database.

What is OLAP?

OLAP stands for Online Analytical Processing. And a class of software programs which are characterized by relatively low frequency of online transactions. Queries are often too complex and involve a bunch of aggregations. 

Find the Constraint information from the table?

There are so many times where user needs to find out the specific constraint information of the table. The following queries are useful, SELECT * From User_Constraints; SELECT * FROM User_Cons_Columns;

Can you get the list of employees with same salary? 

Select distinct e.empid,e.empname,e.salary from employee e, employee e1 where e.salary =e1.salary and e.empid != e1.empid 

What is an alternative for the TOP clause in SQL?

1. ROWCOUNT function 
2. Set rowcount 3
3. Select * from employee order by empid desc Set rowcount 0 

Will the following statement gives an error or 0 as output? SELECT AVG (NULL)

Error. Operand data type NULL is invalid for the Avg operator. 

What is the Cartesian product of the table?

The output of Cross Join is called a Cartesian product. It returns rows combining each row from the first table with each row of the second table. For Example, if we join two tables having 15 and 20 columns the Cartesian product of two tables will be 15×20=300 rows.

What is a schema in SQL?

Our database comprises of a lot of different entities such as tables, stored procedures, functions, database owners and so on. To make sense of how all these different entities interact, we would need the help of schema. So, you can consider schema to be the logical relationship between all the different entities which are present in the database.

Once we have a clear understanding of the schema, this helps in a lot of ways:

  • We can decide which user has access to which tables in the database.
  • We can modify or add new relationships between different entities in the database.

Overall, you can consider a schema to be a blueprint for the database, which will give you the complete picture of how different objects interact with each other and which users have access to different entities.

How to delete a column in SQL?

To delete a column in SQL we will be using DROP COLUMN method:

ALTER TABLE employees
DROP COLUMN age;

We will start off by giving the keywords ALTER TABLE, then we will give the name of the table, following which we will give the keywords DROP COLUMN and finally give the name of the column which we would want to remove.

What is a unique key in SQL?

Unique Key is a constraint in SQL. So, before understanding what exactly is a primary key, let’s understand what exactly is a constraint in SQL. Constraints are the rules enforced on data columns on a table. These are used to limit the type of data that can go into a table. Constraints can either be column level or table level. 

Unique Key: 

Whenever we give the constraint of unique key to a column, this would mean that the column cannot have any duplicate values present in it. In other words, all the records which are present in this column have to be unique.

How to implement multiple conditions using the WHERE clause?

We can implement multiple conditions using AND, OR operators:

SELECT * FROM employees WHERE first_name = ‘Steven’ AND salary <=10000;

In the above command, we are giving two conditions. The condition ensures that we extract only those records where the first name of the employee is ‘Steven’ and the second condition ensures that the salary of the employee is less than $10,000. In other words, we are extracting only those records, where the employee’s first name is ‘Steven’ and this person’s salary should be less than $10,000.

What is the difference between SQL vs PL/SQL?

BASIS FOR COMPARISON SQL PL/SQL
Basic In SQL you can execute a single query or a command at a time. In PL/SQL you can execute a block of code at a time.
Full form Structured Query Language Procedural Language, an extension of SQL.
Purpose It is like a source of data that is to be displayed. It is a language that creates an application that displays data acquired by SQL.
Writes In SQL you can write queries and commands using DDL, DML statements. In PL/SQL you can write a block of code that has procedures, functions, packages or variables, etc.
Use Using SQL, you can retrieve, modify, add, delete, or manipulate the data in the database. Using PL/SQL, you can create applications or server pages that display the information obtained from SQL in a proper format.
Embed You can embed SQL statements in PL/SQL. You can not embed PL/SQL in SQL

What is the difference between SQL having vs where?

S. No. Where Clause Having Clause
1 The WHERE clause specifies the criteria which individual records must meet to be selected by a query. It can be used without the GROUP by clause The HAVING clause cannot be used without the GROUP BY clause
2 The WHERE clause selects rows before grouping The HAVING clause selects rows after grouping
3 The WHERE clause cannot contain aggregate functions The HAVING clause can contain aggregate functions
4 WHERE clause is used to impose a condition on SELECT statement as well as single row function and is used before GROUP BY clause HAVING clause is used to impose a condition on GROUP Function and is used after GROUP BY clause in the query
5 SELECT Column,AVG(Column_nmae)FROM Table_name WHERE Column > value GROUP BY Column_nmae SELECT Columnq, AVG(Coulmn_nmae)FROM Table_name WHERE Column > value GROUP BY Column_nmae Having column_name>or<value

SQL Interview Questions for Experienced

Planning to switch your career to SQL or just need to upgrade your position? Whatever your reason, this section will better prepare you for the SQL interview. We have compiled a set of advanced SQL questions that may be frequently asked during the interview. 

What is SQL injection?

SQL injection is a hacking technique which is widely used by black-hat hackers to steal data from your tables or databases. Let’s say, if you go to a website and give in your user information and password, the hacker would add some malicious code over there such that, he can get the user information and password directly from the database. If your database contains any vital information, it is always better to keep it secure from SQL injection attacks.

What is a trigger in SQL?

A trigger is a stored program in a database which automatically gives responses to an event of DML operations done by inserting, update, or delete. In other words, is nothing but an auditor of events happening across all database tables.

Let’s look at an example of a trigger:

CREATE TRIGGER bank_trans_hv_alert
	BEFORE UPDATE ON bank_account_transaction
	FOR EACH ROW
	begin
	if( abs(:new.transaction_amount)>999999)THEN
    RAISE_APPLICATION_ERROR(-20000, 'Account transaction exceeding the daily deposit on SAVINGS account.');
	end if;
	end;

How to insert multiple rows in SQL?

To insert multiple rows in SQL we can follow the below syntax:

INSERT INTO table_name (column1, column2,column3...)
VALUES
    (value1, value2, value3…..),
    (value1, value2, value3….),
    ...
    (value1, value2, value3);

We start off by giving the keywords INSERT INTO then we give the name of the table into which we would want to insert the values. We will follow it up with the list of the columns, for which we would have to add the values. Then we will give in the VALUES keyword and finally, we will give the list of values.

Here is an example of the same:

INSERT INTO employees (
    name,
    age,
    salary)
VALUES
    (
        'Sam',
        21,
       75000
    ),
    (
        ' 'Matt',
        32,
       85000    ),

    (
        'Bob',
        26,
       90000
    );

In the above example, we are inserting multiple records into the table called employees.

How to find the nth highest salary in SQL?

This is how we can find the nth highest salary in SQL SERVER using TOP keyword:

SELECT TOP 1 salary FROM ( SELECT DISTINCT TOP N salary FROM #Employee ORDER BY salary DESC ) AS temp ORDER BY salary

This is how we can find the nth highest salary in MYSQL using LIMIT keyword:

SELECT salary FROM Employee ORDER BY salary DESC LIMIT N-1, 1

How to copy table in SQL?

We can use the SELECT INTO statement to copy data from one table to another. Either we can copy all the data or only some specific columns.

This is how we can copy all the columns into a new table:

SELECT *
INTO newtable
FROM oldtable
WHERE condition;

If we want to copy only some specific columns, we can do it this way:

SELECT column1, column2, column3, ...
INTO newtable 
FROM oldtable
WHERE condition;

How to add a new column in SQL?

We can add a new column in SQL with the help of alter command:

ALTER TABLE employees ADD COLUMN contact INT(10);

This command helps us to add a new column named as contact in the employees table.

How to use LIKE in SQL?

The LIKE operator checks if an attribute value matches a given string pattern. Here is an example of LIKE operator

SELECT * FROM employees WHERE first_name like ‘Steven’; 

With this command, we will be able to extract all the records where the first name is like “Steven”.

Yes, SQL server drops all related objects, which exists inside a table like constraints, indexex, columns, defaults etc. But dropping a table will not drop views and sorted procedures as they exist outside the table. 

Can we disable a trigger? If yes, How?

Yes, we can disable a single trigger on the database by using “DISABLE TRIGGER triggerName ON<>. We also have an option to disable all the trigger by using, “DISABLE Trigger ALL ON ALL SERVER”.

What is a Live Lock?

A live lock is one where a request for an exclusive lock is repeatedly denied because a series of overlapping shared locks keep interferring. A live lock also occurs when read transactions create a table or page. 

How to fetch alternate records from a table?

Records can be fetched for both Odd and Even row numbers – To display even numbers –

Select employeeId from (Select rowno, employeeId from employee) where mod(rowno,2)=0 

To display odd numbers –

Select employeeId from (Select rowno, employeeId from employee) where mod(rowno,2)=1

Define COMMIT and give an example?

When a COMMIT is used in a transaction, all changes made in the transaction are written into the database permanently.

Example:

BEGIN TRANSACTION; DELETE FROM HR.JobCandidate WHERE JobCandidateID = 20; COMMIT TRANSACTION; 

The above example deletes a job candidate in a SQL server.

Can you join the table by itself? 

A table can be joined to itself using self join, when you want to create a result set that joins records in a table with other records in the same table.

Explain Equi join with an example.

When two or more tables have been joined using equal to operator then this category is called an equi join. Just we need to concentrate on the condition is equal to (=) between the columns in the table.

Example:

Select a.Employee_name,b.Department_name from Employee a,Employee b where a.Department_ID=b.Department_ID

How do we avoid getting duplicate entries in a query?

The SELECT DISTINCT is used to get distinct data from tables using a query. The below SQL query selects only the DISTINCT values from the “Country” column in the “Customers” table:

SELECT DISTINCT Country FROM Customers;

How can you create an empty table from an existing table?

Lets take an example:

Select * into studentcopy from student where 1=2 

Here, we are copying the student table to another table with the same structure with no rows copied.

Write a Query to display odd records from student table?

SELECT * FROM (SELECT *, ROW_NUMBER() OVER (ORDER BY student_no) AS RowID FROM student) WHERE row_id %2!=0

Explain Non-Equi Join with an example?

When two or more tables are joining without equal to condition, then that join is known as Non Equi Join. Any operator can be used here, that is <>,!=,<,>,Between.

Example:

Select b.Department_ID,b.Department_name from Employee a,Department b where a.Department_id <> b.Department_ID;

How can you delete duplicate records in a table with no primary key?

By using the SET ROWCOUNT command. It limits the number of records affected by a command. Let’s take an example, if you have 2 duplicate rows, you would SET ROWCOUNT 1, execute DELETE command and then SET ROWCOUNT 0.

Difference between NVL and NVL2 functions?

Both the NVL(exp1, exp2) and NVL2(exp1, exp2, exp3) functions check the value exp1 to see if it is null. With the NVL(exp1, exp2) function, if exp1 is not null, then the value of exp1 is returned; otherwise, the value of exp2 is returned, but case to the same data type as that of exp1. With the NVL2(exp1, exp2, exp3) function, if exp1 is not null, then exp2 is returned; otherwise, the value of exp3 is returned.

What is the difference between clustered and non-clustered indexes?

  1. Clustered indexes can be read rapidly rather than non-clustered indexes. 
  2. Clustered indexes store data physically in the table or view whereas, non-clustered indexes do not store data in the table as it has separate structure from the data row.

What does this query says? GRANT privilege_name ON object_name TO user_name [WITH GRANT OPTION];

The given syntax indicates that the user can grant access to another user too.

Where MyISAM table is stored?

Each MyISAM table is stored on disk in three files. 

  1. The “.frm” file stores the table definition. 
  2. The data file has a ‘.MYD’ (MYData) extension. 
  3. The index file has a ‘.MYI’ (MYIndex) extension. 

What does myisamchk do?

It compresses the MyISAM tables, which reduces their disk or memory usage.

What is ISAM?

ISAM is abbreviated as Indexed Sequential Access Method. It was developed by IBM to store and retrieve data on secondary storage systems like tapes.

What is Database White box testing?

White box testing includes: Database Consistency and ACID properties Database triggers and logical views Decision Coverage, Condition Coverage, and Statement Coverage Database Tables, Data Model, and Database Schema Referential integrity rules.

What are the different types of SQL sandbox?

There are 3 different types of SQL sandbox: 

  • Safe Access Sandbox: Here a user can perform SQL operations such as creating stored procedures, triggers etc. but cannot have access to the memory as well as cannot create files.
  •  External Access Sandbox: Users can access files without having the right to manipulate the memory allocation.
  • Unsafe Access Sandbox: This contains untrusted codes where a user can have access to memory.

What is Database Black Box Testing?

This testing involves:

  • Data Mapping
  • Data stored and retrieved
  • Use of Black Box testing techniques such as Equivalence Partitioning and Boundary Value Analysis (BVA).

Explain Right Outer Join with Example?

This join is usable, when user wants all the records from Right table (Second table) and only equal or matching records from First or left table. The unmatched records are considered as null records. Example: Select t1.col1,t2.col2….t ‘n’col ‘n.’. from table1 t1,table2 t2 where t1.col(+)=t2.col;

What is a Subquery?

A SubQuery is a SQL query nested into a larger query. Example: SELECT employeeID, firstName, lastName FROM employees WHERE departmentID IN (SELECT departmentID FROM departments WHERE locationID = 2000) ORDER BY firstName, lastName; 

SQL Interview Questions for Developers

How to find duplicate records in SQL?

There are multiple ways to find duplicate records in SQL. Let’s see how can we find duplicate records using group by:

SELECT 
    x, 
    y, 
    COUNT(*) occurrences
FROM z1
GROUP BY
    x, 
    y
HAVING 
    COUNT(*) > 1;

We can also find duplicates in the table using rank:

SELECT * FROM ( SELECT eid, ename, eage, Row_Number() OVER(PARTITION BY ename, eage ORDER By ename) AS Rank FROM employees ) AS X WHERE Rank>1

What is Case WHEN in SQL?

If you have knowledge about other programming languages, then you’d have learnt about if-else statements. You can consider Case WHEN to be analogous to that.

In Case WHEN, there will be multiple conditions and we will choose something on the basis of these conditions.

Here is the syntax for CASE WHEN:

CASE
    WHEN condition1 THEN result1
    WHEN condition2 THEN result2
    WHEN conditionN THEN resultN
    ELSE result
END;

We start off by giving the CASE keyword, then we follow it up by giving multiple WHEN, THEN statements.

How to find 2nd highest salary in SQL?

Below is the syntax to find 2nd highest salary in SQL:

SELECT name, MAX(salary)
  FROM employees
 WHERE salary < (SELECT MAX(salary)
                 FROM employees);

How to delete duplicate rows in SQL?

There are multiple ways to delete duplicate records in SQL.

Below is the code to delete duplicate records using rank:

alter table emp add  sid int identity(1,1)

    delete e
    from  emp e
    inner join
    (select *,
    RANK() OVER ( PARTITION BY eid,ename ORDER BY id DESC )rank
    From emp )T on e.sid=t.sid
    where e.Rank>1

    alter table emp 
    drop  column sno

Below is the syntax to delete duplicate records using groupby and min:

alter table emp add  sno int identity(1,1)
	
	    delete E from emp E
	    left join
	    (select min(sno) sno From emp group by empid,ename ) T on E.sno=T.sno
	    where T.sno is null

	    alter table emp 
	    drop  column sno	

What is cursor in SQL?

Cursors in SQL are used to store database tables. There are two types of cursors:

  • Implicit Cursor
  • Explicit Cursor

Implicit Cursor:

These implicit cursors are default cursors which are automatically created. A user cannot create an implicit cursor.

Explicit Cursor:

Explicit cursors are user-defined cursors. This is the syntax to create explicit cursor:

DECLARE cursor_name CURSOR FOR SELECT * FROM table_name

We start off by giving by keyword DECLARE, then we give the name of the cursor, after that we give the keywords CURSOR FOR SELECT * FROM, finally, we give in the name of the table.

How to create a stored procedure using SQL Server?

If you have worked with other languages, then you would know about the concept of Functions. You can consider stored procedures in SQL to be analogous to functions in other languages. This means that we can store a SQL statement as a stored procedure and this stored procedure can be invoked whenever we want.

This is the syntax to create a stored procedure:

CREATE PROCEDURE procedure_name
AS
sql_statement
GO;

We start off by giving the keywords CREATE PROCEDURE, then we go ahead and give the name of this stored procedure. After that, we give the AS keyword and follow it up with the SQL query, which we want as a stored procedure. Finally, we give the GO keyword.

Once, we create the stored procedure, we can invoke it this way:

EXEC procedure_name;

We will give in the keyword EXEC and then give the name of the stored procedure.

Let’s look at an example of a stored procedure:

CREATE PROCEDURE employee_location @location nvarchar(20)
AS
SELECT * FROM employees WHERE location = @location
GO;

In the above command, we are creating a stored procedure which will help us to extract all the employees who belong to a particular location.

EXEC employee_location @location = 'Boston';

With this, we are extracting all the employees who belong to Boston.

How to create an index in SQL?

We can create an index using this command:

CREATE INDEX index_name
ON table_name (column1, column2, column3 ...);

We start off by giving the keywords CREATE INDEX and then we will follow it up with the name of the index, after that we will give the ON keyword. Then, we will give the name of the table on which we would want to create this index. Finally, in parenthesis, we will list out all the columns which will have the index. Let’s look at an example:

CREATE INDEX salary
ON Employees (Salary);

In the above example, we are creating an index called a salary on top of the ‘Salary’ column of the ‘Employees’ table.

Now, let’s see how can we create a unique index:

CREATE UNIQUE INDEX index_name
ON table_name (column1, column2,column3 ...);

We start off with the keywords CREATE UNIQUE INDEX, then give in the name of the index, after that, we will give the ON keyword and follow it up with the name of the table. Finally, in parenthesis, we will give the list of the columns which on which we would want this unique index.

How to change the column data type in SQL?

We can change the data type of the column using the alter table. This will be the command:

ALTER TABLE table_name
MODIFY COLUMN column_name datatype;

We start off by giving the keywords ALTER TABLE, then we will give in the name of the table. After that, we will give in the keywords MODIFY COLUMN. Going ahead, we will give in the name of the column for which we would want to change the datatype and finally we will give in the data type to which we would want to change.

Difference between SQL and NoSQL databases?

SQL stands for structured query language and is majorly used to query data from relational databases. When we talk about a SQL database, it will be a relational database. 

But when it comes to the NoSQL databases, we will be working with non-relational databases.

SQL Joins Interview Questions

How to change column name in SQL?

The command to change the name of a column is different in different RDBMS.

This is the command to change the name of a column in MYSQL:

ALTER TABLE Customer CHANGE Address Addr char(50);

IN MYSQL, we will start off by using the ALTER TABLE keywords, then we will give in the name of the table. After that, we will use the CHANGE keyword and give in the original name of the column, following which we will give the name to which we would want to rename our column.

This is the command to change the name of a column in ORACLE:

ALTER TABLE Customer RENAME COLUMN Address TO Addr;

In ORACLE, we will start off by using the ALTER TABLE keywords, then we will give in the name of the table. After that, we will use the RENAME COLUMN keywords and give in the original name of the column, following which we will give the TO keyword and finally give the name to which we would like to rename our column.

When it comes to SQL Server, it is not possible to rename the column with the help of ALTER TABLE command, we would have to use sp_rename.

What is a view in SQL?

A view is a database object that is created using a Select Query with complex logic, so views are said to be a logical representation of the physical data, i.e Views behave like a physical table and users can use them as database objects in any part of SQL queries.

Let’s look at the types of Views:

  • Simple View
  • Complex View
  • Inline View
  • Materialized View

Simple View:

Simple views are created with a select query written using a single table. Below is the command to create a simple view:

Create VIEW Simple_view as Select * from BANK_CUSTOMER ;

Complex View:

Create VIEW Complex_view as SELECT bc.customer_id , ba.bank_account From Bank_customer bc JOIN Bank_Account ba Where bc.customer_id = ba.customer_id And ba.balance > 300000

Inline View:

A subquery is also called an inline view if and only if it is called in FROM clause of a SELECT query.

SELECT * FROM ( SELECT bc.customer_id , ba.bank_account From Bank_customer bc JOIN Bank_Account ba Where bc.customer_id = ba.customer_id And ba.balance > 300000)
view in SQL

How to drop a column in SQL?

To drop a column in SQL, we will be using this command:

ALTER TABLE employees
DROP COLUMN gender;

We will start off by giving the keywords ALTER TABLE, then we will give the name of the table, following which we will give the keywords DROP COLUMN and finally give the name of the column which we would want to remove.

How to use BETWEEN in SQL?

The BETWEEN operator checks an attribute value within a range. Here is an example of BETWEEN operator:

SELECT * FROM employees WHERE salary between 10000 and 20000;

With this command, we will be able to extract all the records where the salary of the employee is between 10000 and 20000.

Advanced SQL Interview Questions

What are the subsets of SQL?

  • DDL (Data Definition Language): Used to define the data structure it consists of the commands like CREATE, ALTER, DROP, etc. 
  • DML (Data Manipulation Language): Used to manipulate already existing data in the database, commands like SELECT, UPDATE, INSERT 
  • DCL (Data Control Language): Used to control access to data in the database, commands like GRANT, REVOKE.

Difference between CHAR and VARCHAR2 datatype in SQL?

CHAR is used to store fixed-length character strings, and VARCHAR2 is used to store variable-length character strings.

How to sort a column using a column alias?

By using the column alias in the ORDER BY instead of where clause for sorting

Difference between COALESCE() & ISNULL() ?

COALESCE() accepts two or more parameters, one can apply 2 or as many parameters but it returns only the first non NULL parameter. 

ISNULL() accepts only 2 parameters. 

The first parameter is checked for a NULL value, if it is NULL then the 2nd parameter is returned, otherwise, it returns the first parameter.

What is “Trigger” in SQL?

A trigger allows you to execute a batch of SQL code when an insert,update or delete command is run against a specific table as Trigger is said to be the set of actions that are performed whenever commands like insert, update or delete are given. 

Write a Query to display employee details along with age.

SELECT * DATEDIFF(yy, dob, getdate()) AS 'Age' FROM employee

Write a Query to display employee details along with age?

SELECT SUM(salary) FROM employee

Write an SQL query to get the third maximum salary of an employee from a table named employee_table.

SELECT TOP 1 salary FROM ( SELECT TOP 3 salary FROM employee_table ORDER BY salary DESC ) AS emp ORDER BY salary ASC; 

What are aggregate and scalar functions?

Aggregate functions are used to evaluate mathematical calculations and return single values. This can be calculated from the columns in a table. Scalar functions return a single value based on input value. 

Example -. Aggregate – max(), count – Calculated with respect to numeric. Scalar – UCASE(), NOW() – Calculated with respect to strings.

What is a deadlock?

It is an unwanted situation where two or more transactions are waiting indefinitely for one another to release the locks. 

Explain left outer join with example.

Left outer join is useful if you want all the records from the left table(first table) and only matching records from 2nd table. The unmatched records are null records. Example: Left outer join with “+” operator Select t1.col1,t2.col2….t ‘n’col ‘n.’. from table1 t1,table2 t2 where t1.col=t2.col(+);

What is SQL injection?

SQL injection is a code injection technique used to hack data-driven applications.

What is a UNION operator?

The UNION operator combines the results of two or more Select statements by removing duplicate rows. The columns and the data types must be the same in the SELECT statements.

Explain SQL Constraints.

SQL Constraints are used to specify the rules of data type in a table. They can be specified while creating and altering the table. The following are the constraints in SQL: NOT NULL CHECK DEFAULT UNIQUE PRIMARY KEY FOREIGN KEY

What is the ALIAS command?

This command provides another name to a table or a column. It can be used in the WHERE clause of a SQL query using the “as” keyword. 

What are Group Functions? Why do we need them?

Group functions work on a set of rows and return a single result per group. The popularly used group functions are AVG, MAX, MIN, SUM, VARIANCE, and COUNT.

How can dynamic SQL be executed?

  • By executing the query with parameters 
  • By using EXEC 
  • By using sp_executesql

What is the usage of NVL() function?

This function is used to convert the NULL value to the other value.

Write a Query to display employee details belongs to ECE department?

SELECT EmpNo, EmpName, Salary FROM employee WHERE deptNo in (select deptNo from dept where deptName = ‘ECE’)

What are the main differences between #temp tables and @table variables and which one is preferred?

1. SQL server can create column statistics on #temp tables. 

2. Indexes can be created on #temp tables 

3. @table variables are stored in memory up to a certain threshold

What is CLAUSE?

SQL clause is defined to limit the result set by providing conditions to the query. This usually filters some rows from the whole set of records. Example – Query that has WHERE condition.

What is a recursive stored procedure?

A stored procedure calls by itself until it reaches some boundary condition. This recursive function or procedure helps programmers to use the same set of code any number of times.

What does the BCP command do?

The Bulk Copy is a utility or a tool that exports/imports data from a table into a file and vice versa. 

What is a Cross Join?

In SQL cross join, a combination of every row from the two tables is included in the result set. This is also called cross product set. For example, if table A has ten rows and table B has 20 rows, the result set will have 10 * 20 = 200 rows provided there is a NOWHERE clause in the SQL statement.

Which operator is used in query for pattern matching?

LIKE operator is used for pattern matching, and it can be used as- 1. % – Matches zero or more characters. 2. _(Underscore) – Matching exactly one character.

Write a SQL query to get the current date?

SELECT CURDATE();

State the case manipulation functions in SQL?

  • LOWER: converts all the characters to lowercase.
  • UPPER: converts all the characters to uppercase. 
  • INITCAP: converts the initial character of each word to uppercase

How to add a column to an existing table?

ALTER TABLE Department ADD (Gender, M, F)

Define lock escalation?

A query first takes the lowest level lock possible with the smallest row level. When too many rows are locked, the lock is escalated to a range or page lock. If too many pages are locked, it may escalate to a table lock. 

How to store Videos inside SQL Server table?

By using FILESTREAM datatype, which was introduced in SQL Server 2008.

State the order of SQL SELECT?

The order of SQL SELECT clauses is: SELECT, FROM, WHERE, GROUP BY, HAVING, ORDER BY. Only the SELECT and FROM clauses are mandatory.

What is the difference between IN and EXISTS?

IN: Works on List result set Doesn’t work on subqueries resulting in Virtual tables with multiple columns Compares every value in the result list.

Exists: Works on Virtual tables Is used with co-related queries Exits comparison when the match is found

How do you copy data from one table to another table?

INSERT INTO table2 (column1, column2, column3, …) SELECT column1, column2, column3, … FROM table1 WHERE condition;

List the ACID properties that make sure that the database transactions are processed

ACID (Atomicity, Consistency, Isolation, Durability) is a set of properties that guarantee that database transactions are processed reliably. 

What will be the output of the following Query, provided the employee table has 10 records? 

BEGIN TRAN TRUNCATE TABLE Employees ROLLBACK SELECT * FROM Employees

This query will return 10 records as TRUNCATE was executed in the transaction. TRUNCATE does not itself keep a log but BEGIN TRANSACTION keeps track of the TRUNCATE command.

What do you mean by Stored Procedures? How do we use it?

A stored procedure is a collection of SQL statements that can be used as a function to access the database. We can create these stored procedures earlier before using it and can execute them wherever required by applying some conditional logic to it. Stored procedures are also used to reduce network traffic and improve performance.

What does GRANT command do?

This command is used to provide database access to users other than the administrator in SQL privileges.

What does the First normal form do?

First Normal Form (1NF): It removes all duplicate columns from the table. It creates a table for related data and identifies unique column values.

How to add e record to the table?

INSERT syntax is used to add a record to the table. INSERT into table_name VALUES (value1, value2..);

What are the different tables present in MySQL?

There are 5 tables present in MYSQL.

  • MyISAM 
  • Heap 
  • Merge 
  • INNO DB 
  • ISAM

What is BLOB and TEXT in MySQL?

BLOB stands for the large binary objects. It is used to hold a variable amount of data. TEXT is a case-insensitive BLOB. TEXT values are non-binary strings (character strings). 

What is the use of mysql_close()?

Mysql_close() cannot be used to close the persistent connection. Though it can be used to close a connection opened by mysql_connect().

Write a query to find out the data between ranges?

In day-to-day activities, the user needs to find out the data between some range. To achieve this user needs to use Between..and operator or Greater than and less than the operator. 

Query 1: Using Between..and operator
Select * from Employee where salary between 25000 and 50000;
Query 2: Using operators (Greater than and less than)
Select * from Employee where salary >= 25000 and salary <= 50000;

How to calculate the number of rows in a table without using the count function?

There are so many system tables which are very important. Using the system table user can count the number of rows in the table. following query is helpful in that case, Select table_name, num_rows from user_tables where table_name=’Employee’;

What is wrong with the following query? SELECT empName FROM employee WHERE salary <> 6000

The following query will not fetch a record with the salary of 6000 but also will skip the record with NULL. 

Will the following statements execute? if yes what will be output? SELECT NULL+1 SELECT NULL+’1′

Yes, no error. The output will be NULL. Performing any operation on NULL will get the NULL result.

SQL Server Interview Questions

What is an SQL server?

SQL server has stayed on top as one of the most popular database management products ever since its first release in 1989 by Microsoft Corporation. The product is used across industries to store and process large volumes of data. It was primarily built to store and process data that is built on a relational model of data. 

SQL Server is widely used for data analysis and also scaling up of data. SQL Server can be used in conjunction with Big Data tools such as Hadoop

SQL Server can be used to process data from various data sources such as Excel, Table, .Net Framework application, etc.

How to install SQL Server?

  • Click on the below SQL Server official release link to access the latest version: https://www.microsoft.com/en-in/sql-server/sql-server-downloads
  • Select the type of SQL Server edition that you want to install. SQL Server can be used on a Cloud Platform or as an open-source edition(Express or Developer) in your local computer system. 
  • Click on the Download Now button.
  • Save the .exe file on your system. Right-click on the .exe file and click on Open.
  • Click on ‘Yes’ to allow the changes to be made on your system and have SQL Server Installed.
  • Once the installation is complete, restart your system, if required, and launch the SQL Server Management Studio application from the START menu.

How to create a stored procedure in SQL Server?

A Stored Procedure is nothing but a frequently used SQL query. Queries such as a SELECT query, which would often be used to retrieve a set of information many times within a database, can be saved as a Stored Procedure. The Stored Procedure, when called, executes the SQL query saved within the Stored Procedure.

Syntax to create a Stored Proc:

CREATE PROCEDURE PROCEDURE_NAME
AS
SQL_QUERY (GIVE YOUR OFTEN USED QUERY HERE)
GO;

Stored procedures can be user-defined or built-in. Various parameters can be passed onto a Stored Procedure.

How to install SQL Server 2008?

  • Click on the below SQL Server official release link: https://www.microsoft.com/en-in/sql-server/sql-server-downloads
  • Click on the search icon and type in – SQL Server 2008 download
  • Click on the result link to download and save SQL Server 2008.
  • Select the type of SQL Server edition that you want to install. SQL Server can be used on a Cloud Platform or as an open-source edition(Express or Developer) in your local computer system.
  • Click on the Download Now button.
  • Save the .exe file on your system. Right-click on the .exe file and click on Open.
  • Click on ‘Yes’ to allow the changes to be made on your system and have SQL Server installed.
  • Once the installation is complete, restart your system, if required, and launch the SQL Server Management Studio application.

How to install SQL Server 2017?

  • Click on the below SQL Server official release link: https://www.microsoft.com/en-in/sql-server/sql-server-downloads
  • Click on the search icon and type in – SQL Server 2017 download
  • Click on the result link to download and save SQL Server 2017.
  • Select the type of SQL Server edition that you want to install. SQL Server can be used on a Cloud Platform or as an open-source edition(Express or Developer) in your local computer system.
  • Click on the Download Now button.
  • Save the .exe file on your system. Right-click on the .exe file and click on Open.
  • Click on ‘Yes’ to allow the changes to be made on your system and have SQL Server installed.
  • Once the installation is complete, restart your system, if required, and launch the SQL Server Management Studio application from the START menu.

How to restore the database in SQL Server?

Launch the SQL Server Management Studio application and from the Object Explorer window pane, right-click on Databases and click on Restore. This would automatically restore the database.

How to install SQL Server 2014?

  • Click on the below SQL Server official release link: https://www.microsoft.com/en-in/sql-server/sql-server-downloads
  • Click on the search icon and type in – SQL Server 2014 download
  • Click on the result link to download and save SQL Server 2014.
  • Select the type of SQL Server edition that you want to install. SQL Server can be used on a Cloud Platform or as an open-source edition(Express or Developer) in your local computer system.
  • Click on the Download Now button.
  • Save the .exe file on your system. Right-click on the .exe file and click on Open.
  • Click on ‘Yes’ to allow the changes to be made on your system and have SQL Server Installed.
  • Once the installation is complete, restart your system, if required, and launch the SQL Server Management Studio application from the START menu.

How to get the connection string from SQL Server?

Launch the SQL Server Management Studio. Go to the Database for which you require the Connection string. Right-click on the database and click on Properties. In the Properties window that is displayed, you can view the Connection String property.

Connection strings help connect databases to another staging database or any external source of data.

How to install SQL Server 2012?

  • Click on the below SQL Server official release link: https://www.microsoft.com/en-in/sql-server/sql-server-downloads
  • Click on the search icon and type in – SQL Server 2012 download
  • Click on the result link to download and save SQL Server 2012.
  • Select the type of SQL Server edition that you want to install. SQL Server can be used on a Cloud Platform or as an open-source edition(Express or Developer) in your local computer system.
  • Click on the Download Now button.
  • Save the .exe file on your system. Right-click on the .exe file and click on Open.
  • Click on ‘Yes’ to allow the changes to be made on your system and have SQL Server Installed.
  • Once the installation is complete, restart your system, if required, and launch the SQL Server Management Studio application from the START menu.

What is cte in SQL Server?

CTEs are Common Table Expressions that are used to create temporary result tables from which data can be retrieved/ used. The standard syntax for a CTE with a SELECT statement is:

WITH RESULT AS 
(SELECT COL1, COL2, COL3
FROM EMPLOYEE)
SELECT COL1, COL2 FROM RESULT
CTEs can be used with Insert, Update or Delete statements as well.

Few examples of CTEs are given below:

Query to find the 10 highest salaries.

with result as 

(select distinct salary, dense_rank() over (order by salary desc) as salary rank from employees)
select result. salary from result where the result.salaryrank = 10

 

Query to find the 2nd highest salary

with the result as 

(select distinct salary, dense_rank() over (order by salary desc) as salaryrank from employees)
select result. salary from result where the result.salaryrank = 2

In this way, CTEs can be used to find the nth highest salary within an organisation.

How to change the SQL Server password?

Launch your SQL Server Management Studio. Click on the Database connection for which you want to change the login password. Click on Security from the options that get displayed. 

Click on Logins and open your database connection. Type in the new password for login and click on ‘OK’ to apply the changes. 

How to delete duplicate records in SQL Server?

Select the duplicate records in a table HAVING COUNT(*)>1 

Add a delete statement to delete the duplicate records.

Sample Query to find the duplicate records in a table-

(SELECT COL1, COUNT(*) AS DUPLICATE
FROM EMPLOYEE
GROUP BY COL1
HAVING COUNT(*) > 1)

How to uninstall SQL Server?

In Windows 10, go to the START menu and locate the SQL Server.

Right-click and select uninstall to uninstall the application.

How to check SQL Server version?

You can run the below query to view the current version of SQL Server that you are using.

How to rename column name in SQL Server?

From the Object Explorer window pane, go to the table where the column is present and choose Design. Under the Column Name, select the name you want to rename and enter the new name. Go to the File menu and click Save. 

What is the stored procedure in SQL Server?

A Stored Procedure is nothing but a frequently used SQL query. Queries such as a SELECT query, which would often be used to retrieve a set of information many times within a database, can be saved as a Stored Procedure. The Stored Procedure, when called, executes the SQL query saved within the Stored Procedure.

Syntax to create a Stored Proc:

CREATE PROCEDURE PROCEDURE_NAME
AS
SQL_QUERY (GIVE YOUR OFTEN USED QUERY HERE)
GO;

You can execute the Stored Proc by using the command Exec Procedure_Name;

How to create a database in SQL Server?

After installing the required version of SQL Server, it is easy to create new databases and maintain them. 

  1. Launch the SQL Server Management Studio
  2. In the Object Explorer window pane, right-click on Databases and select ‘New Database’
  3. Enter the Database Name and click on ‘Ok’.
  4. Voila! Your new database is ready for use.

What is an index in SQL Server?

Indexes are database objects which help in retrieving records quickly and more efficiently. Column indexes can be created on both Tables and Views. By declaring a Column as an index within a table/ view, the user can access those records quickly by executing the index. Indexes with more than one column are called Clustered indexes.

Syntax:

CREATE INDEX INDEX_NAME
ON TABLE_NAME(COL1, COL2);

The syntax to drop an Index is DROP INDEX INDEX_NAME;

Indexes are known to improve the efficiency of SQL Select queries. 

How to create the table in SQL Server?

Tables are the fundamental storage objects within a database. A table is usually made up of 

Rows and Columns. The below syntax can be used to create a new table with 3 columns.

CREATE TABLE TABLE_NAME(
COLUMN1 DATATYPE, 
COLUMN2 DATATYPE, 
COLUMN3 DATATYPE
);

Alternatively, you can right-click on Table in the Object Explorer window pane and select ‘New -> Table’.

You can also define the type of Primary/ Foreign/ Check constraint when creating a table.

How to connect to SQL Server?

  • Launch the SQL Server Management Studio from the START menu.
  • In the dialogue box shown below, select the Server Type as Database Engine and Server Name as the name of your laptop/ desktop system.
  • Select the appropriate Authentication type and click on the Connect button.
  • A secure connection would be established, and the list of the available Databases will be loaded in the Object Explorer window pane.

How to delete duplicate rows in SQL Server?

Select the duplicate records in a table HAVING COUNT(*)>1 

Add a delete statement to delete the duplicate records.

Sample Query to find the duplicate records in a table –

(SELECT COL1, COUNT(*) AS DUPLICATE
FROM EMPLOYEE
GROUP BY COL1
HAVING COUNT(*) > 1);

How to download SQL Server?

The Express and Developer versions (open-source versions) of the latest SQL Server release can be downloaded from the official Microsoft website. The link is given below for reference.
https://www.microsoft.com/en-in/sql-server/sql-server-downloads

How to connect SQL Server management studio to the local database?

  • Launch the SQL Server Management Studio from the START menu.
  • In the dialogue box shown below, select the Server Type as Database Engine and Server Name as the name of your laptop/ desktop system and click on the Connect button.
  • Select the Authentication as ‘Windows Authentication.
  • A secure connection would be established, and the list of the available Databases will be loaded in the Object Explorer window pane.

How to download SQL Server 2014?

  • Both the Express and Developer versions (free editions) of SQL Server can be downloaded from the official Microsoft website. The link is given below for reference.
  • Click on the link below: https://www.microsoft.com/en-in/sql-server/sql-server-downloads
  • Click on the search icon and type in – SQL Server 2014 download
  • Click on the result link to download and save SQL Server 2014.

How to uninstall SQL Server 2014?

From the START menu, type SQL Server. Right-click on the app and select uninstall to uninstall the application from your system. Restart the system, if required, for the changes to get affected. 

How to find server names in SQL Server?

Run the query SELECT @@version; to find the version and name of the SQL Server you are using. 

How to start SQL Server?

Launch the SQL Server Management Studio from the START menu. Login using Windows Authentication. In the Object Explorer window pane, you can view the list of databases and corresponding objects. 

What is the case when in SQL Server?

Case When statements in SQL are used to run through many conditions and to return a value when one such condition is met. If none of the conditions is met in the When statements, then the value mentioned in the Else statement is returned. 

Syntax:

CASE
WHEN CONDITION1 THEN RESULT1

WHEN CONDITION2 THEN RESULT2

ELSE
RESULT
END;

Sample query:

HOW MANY HEAD OFFICES/ BRANCHES ARE THERE IN CANADA

select 
sum ( 
case 
when region_id >=  5 AND region_id <= 7 then  
1
else 
0
end ) as Canada
from company_regions;
Nested CASE statement:
SELECT
SUM (
CASE
WHEN rental_rate = 0.99 THEN
1
ELSE
0
END
) AS "Mass",
SUM (
CASE
WHEN rental_rate = 2.99 THEN
1
ELSE
0
END
) AS "Economic",
SUM (
CASE
WHEN rental_rate = 4.99 THEN
1
ELSE
0
END
) AS " Luxury"
FROM
film;

How to install SQL Server management studio?

Launch Google and in the Search toolbar, type in SQL Server Management Studio download. 

Go to the routed website and click on the link to download. Once the download is complete, open the .exe file to install the content of the file. Once the installation is complete, refresh or restart the system, as required.

Alternatively, once SQL Server is installed and launched, it will prompt the user with an option to launch SQ Server Management Studio. 

How to write a stored procedure in SQL Server?

A Stored Procedure is nothing but a frequently used SQL query. Queries such as a SELECT query, which would often be used to retrieve a set of information many times within a database, can be saved as a Stored Procedure. The Stored Procedure, when called, executes the SQL query saved within the Stored Procedure.

Syntax to create a Stored Proc:

CREATE PROCEDURE PROCEDURE_NAME
AS
SQL_QUERY (GIVE YOUR OFTEN USED QUERY HERE)
GO;

You can execute the Stored Proc by using the command Exec Procedure_Name;

How to open SQL Server?

Launch the SQL Server Management Studio from the START menu. Login using Windows Authentication. In the Object Explorer window pane, you can view the list of databases and corresponding objects. 

How to use SQL Server?

SQL Server is used to retrieve and process various data that is built on a relational model.

Some of the common actions that can be taken on the data are CREATE, DELETE, INSERT, UPDATE, SELECT, REVOKE, etc.

SQL Server can also be used to import and export data from different data sources. SQL Server can also be connected to various other databases/ .Net frameworks using Connection Strings.

SQL Server can also be used in conjunction with Big Data tools like Hadoop. 

What is a function in SQL Server?

Functions are pre-written codes that return a value and which help the user achieve a particular task concerning viewing, manipulating, and processing data.

Examples of a few functions are:

AGGREGATE FUNCTIONS:

  • MIN()- Returns the minimum value
  • MAX()- Returns the maximum value
  • AVG()- Returns the average value
  • COUNT()

STRING FUNCTIONS:

  • COALESCE()
  • CAST()
  • CONCAT()
  • SUBSTRING()

DATE FUNCTIONS:

  • GETDATE()
  • DATEADD()
  • DATEDIFF()

There are many types of functions such as Aggregate Functions, Date Functions, String Functions, Mathematical functions, etc.

How to find nth highest salary in SQL Server without using a subquery

Query to find the 10 highest salaries. For up-gradation of the b10 band.

with result as 

(select distinct salary, dense_rank() over (order by salary desc) as salaryrank from employees)
select result.salary from result where result.salaryrank = 10

Query to find the 2nd highest salary

with the result as 

(select distinct salary, dense_rank() over (order by salary desc) as salary rank from employees)
select result.salary from result where result.salaryrank = 2

In this way, by replacing the salary rank value, we can find the nth highest salary in any organisation.

How to install SQL Server in Windows 10?

Click on the below SQL Server official release link: https://www.microsoft.com/en-in/sql-server/sql-server-downloads
Click on the search icon and type in - SQL Server 2012 download
Click on the result link to download and save SQL Server 2012.
Select the type of the SQL Server edition that you want to install. SQL Server can be used on a Cloud Platform or as an open-source edition(Express or Developer) in your local computer system.
Click on the Download Now button.
Save the .exe file on your system. Right-click on the .exe file and click on Open.
Click on ‘Yes’ to allow the changes to be made on your system and have SQL Server Installed

How to create a temp table in SQL Server?

Temporary tables can be used to retain the structure and a subset of data from the original table from which they were derived. 

Syntax:

SELECT COL1, COL2
INTO TEMPTABLE1
FROM ORIGTABLE;

Temporary tables do not occupy any physical memory and can be used to retrieve data faster.

PostgreSQL Interview Questions

What is PostgreSQL?

PostgreSQL is one of the most widely and popularly used languages for Object-Relational Database Management systems. It is mainly used for large web applications. It is an open-source, object-oriented, -relational database system. It is extremely powerful and enables users to extend any system without problem. It extends and uses the SQL language in combination with various features for safely scaling and storage of intricate data workloads.

List different datatypes of PostgreSQL?

Listed below are some of the new data types in PostgreSQL

  • UUID
  • Numeric types
  • Boolean
  • Character types
  • Temporal types
  • Geometric primitives
  • Arbitrary precision numeric
  • XML
  • Arrays etc

What are the Indices of PostgreSQL?

Indices in PostgreSQL allow the database server to find and retrieve specific rows in a given structure. Examples are B-tree, hash, GiST, SP-GiST, GIN and BRIN.  Users can also define their indices in PostgreSQL. However, indices add overhead to the data manipulation operations and are seldom used

What are tokens in PostgreSQL?

Tokens in PostgreSQL act as the building blocks of a source code. They are composed of various special character symbols. Commands are composed of a series of tokens and terminated by a semicolon(“;”). These can be a constant, quoted identifier, other identifiers, keyword or a constant. Tokens are usually separated by whitespaces.

How to create a database in PostgreSQL?

Databases can be created using 2 methods 

  • First is the CREATE DATABASE SQL Command

We can create the database by using the syntax:-

CREATE DATABASE <dbname>;
  • The second is by using the createdb command

We can create the database by using the syntax:-

createdb [option...] <dbname> 

Are you an aspiring SQL Developer? A career in SQL has seen an upward trend in 2022, and you can be a part of the ever-so-growing community. So, if you are ready to indulge yourself in the pool of knowledge and be prepared for the upcoming SQL interview, then you are at the right place. […]

The post Top 180+ SQL Interview Questions and Answers in 2022 appeared first on GreatLearning Blog: Free Resources what Matters to shape your Career!.

Various options can be taken by the createDB command based on the use case.

How to create a table in PostgreSQL?

You can create a new table by specifying the table name, along with all column names and their types:

CREATE TABLE [IF NOT EXISTS] table_name (
column1 datatype(length) column_contraint,
column2 datatype(length) column_contraint,
.
.
.
columnn datatype(length) column_contraint,
table_constraints
);

How can we change the column datatype in PostgreSQL?

The column the data type can be changed in PostgreSQL by using the ALTER TABLE command:

ALTER TABLE table_name
ALTER COLUMN column_name1 [SET DATA] TYPE new_data_type,
ALTER COLUMN column_name2 [SET DATA] TYPE new_data_type,
...;

Compare ‘PostgreSQL’ with ‘MongoDB’

PostgreSQL MongoDB
PostgreSQL is an SQL database where data is stored as tables, with structured rows and columns. It supports concepts like referential integrity entity-relationship and JOINS. PostgreSQL uses SQL as its querying language. PostgreSQL supports vertical scaling. This means that you need to use big servers to store data. This leads to a requirement of downtime to upgrade. It works better if you require relational databases in your application or need to run complex queries that test the limit of SQL. MongoDB, on the other hand, is a NoSQL database. There is no requirement for a schema, therefore it can store unstructured data. Data is stored as BSON documents and the document’s structure can be changed by the user. MongoDB uses JavaScript for querying. It supports horizontal scaling, as a result of which additional servers can be added as per the requirement with minimal to no downtime.  It is appropriate in a use case that requires a highly scalable distributed database that stores unstructured data

What is Multi-Version concurrency control in PostgreSQL?

MVCC or better known as Multi-version concurrency control is used to implement transactions in PostgreSQL. It is used to avoid unwanted locking of a database in the system. while querying a database each transaction sees a version of the database. This avoids viewing inconsistencies in the data, and also provides transaction isolation for every database session. MVCC locks for reading data do not conflict with locks acquired for

How do you delete the database in PostgreSQL?

Databases can be deleted in PostgreSQL using the syntax

DROP DATABASE [IF EXISTS] <database_name>;

Please note that only databases having no active connections can be dropped.

What does a schema contain?

  • Schemas are a part of the database that contains tables. They also contain other kinds of named objects, like data types, functions, and operators.
  • The object names can be used in different schemas without conflict; Unlike databases, schemas are separated more flexibly. This means that a user can access objects in any of the schemas in the database they are connected to, till they have privileges to do so.
  • Schemas are highly beneficial when there is a need to allow many users access to one database without interfering with each other. It helps in organizing database objects into logical groups for better manageability. Third-party applications can be put into separate schemas to avoid conflicts based on names.

What is the square root operator in PostgreSQL?

It is denoted by ‘|/” and returns the square root of a number. Its syntax is

Egs:- Select |/16

How are the stats updated in Postgresql?

To update statistics in PostgreSQL a special function called an explicit ‘vacuum’ call is made. Entries in pg_statistic are updated by the ANALYZE and VACUUM ANALYZE commands

What Is A Candid?

The CTIDs field exists in every PostgreSQL table. It is unique for every record of a table and exactly shows the location of a tuple in a particular table. A logical row’s CTID changes when it is updated, thus it cannot be used as a permanent row identifier. However, it is useful when identifying a row within a transaction when no update is expected on the data item.

What is Cube Root Operator (||/) in PostgreSQL?

It is denoted by ‘|/” and returns the square root of a number. Its syntax is

Egs:- Select |/16

Explain Write-Ahead Logging?

Write-ahead logging is a method to ensure data integrity. It is a protocol that ensures writing the actions as well as changes into a transaction log. It is known to increase the reliability of databases by logging changes before they are applied or updated onto the database. This provides a backup log for the database in case of a crash.

What is a non-clustered index?

A non-clustered index in PostgreSQL is a simple index, used for fast retrieval of data, with no certainty of the uniqueness of data. It also contains pointers to locations where other parts of data are stored

How is security ensured in PostgreSQL?

PostgreSQL uses 2 levels of security

  • Network-level security uses Unix Domain sockets, TCP/IP sockets, and firewalls.
  • Transport-level security which uses SSL/TLS to enable secure communication with the database
  • Database-level security with features like roles and permissions, row-level security (RLS), and auditing.

Frequently Asked Questions in SQL

1. How do I prepare for the SQL interview?

There are many sources online that can help you prepare for an SQL interview. You can go through brief tutorials and free online courses on SQL (eg.: SQL basics on Great Learning Academy) to revise your knowledge of SQL. You can also practice projects to help you with practical aspects of the language. Lastly, there are many blogs such as this that list out all the probable questions that an interviewer might ask. 

2. What are the 5 basic SQL commands?

The five basic SQL commands are:

  • Data Definition Language (DDL)
  • Data Manipulation Language (DML)
  • Data Control Language (DCL)
  • Transaction Control Language (TCL)
  • Data Query Language (DQL)

3. What are basic SQL skills?

SQL is a vast topic and there is a lot to learn. But the most basic skills that an SQL professional should know are:

  • How to structure a database
  • Managing a database
  • Authoring SQL statements and clauses
  • Knowledge of popular database systems such as MySQL
  • Working knowledge of PHP
  • SQL data analysis
  • Creating a database with WAMP and SQL

4. How can I practice SQL?

There are some platforms available online that can help you practice SQL such as SQL Fiddle, SQLZOO, W3resource, Oracle LiveSQL, DB-Fiddle, Coding Groud, GitHub and others. Also take up a Oracle SQL to learn more.

5. Where can I practice SQL questions?

There are some platforms available online that can help you practice SQL such as SQL Fiddle, SQLZOO, W3resource, Oracle LiveSQL, DB-Fiddle, Coding Groud, GitHub and others. 

You can also refer to articles and blogs online that list the most important SQL interview questions for preparation.

6. What is the most common SQL command?

Some of the most common SQL commands are:

  • CREATE DATABASE 
  • ALTER DATABASE
  • CREATE TABLE
  • ALTER TABLE 
  • DROP TABLE
  • CREATE INDEX
  • DROP INDEX

7. How are SQL commands classified?

SQL Commands are classified under four categories, i.e.,

  • Data Definition Language (DDL)
  • Data Query Language (DQL)
  • Data Manipulation Language (DML)
  • Data Control Language (DCL)

8. What are basic SQL commands?

Basic SQL commands are:

  • CREATE DATABASE 
  • ALTER DATABASE
  • CREATE TABLE
  • ALTER TABLE 
  • DROP TABLE
  • CREATE INDEX
  • DROP INDEX

9. Is SQL coding?

Yes, SQL is a coding language/ programming language that falls under the category of domain-specific programming language. It is used to access relational databases such as MySQL.

10. What is SQL example?

SQL helps you update, delete, and request information from databases. Some of the examples of SQL are in the form of the following statements:

  • SELECT 
  • INSERT 
  • UPDATE
  • DELETE
  • CREATE DATABASE
  • ALTER DATABASE 

11. What is SQL code used for?

SQL code is used to access and communicate with a database. It helps in performing tasks such as updating and retrieving data from the databases.

To Conclude

For anyone who is well-versed with SQL knows that it is the most widely used Database language. Thus, the most essential part to learn is SQL for Data Science to power ahead in your career.

Wondering where to learn the highly coveted in demand skills for free? Check out the courses on Great Learning Academy. Enroll to any course, learn the in demand skill and get your free certificate. Hurry!

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