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Top 23+ SQL Server Interview Questions and Answers

25/Oct/2021 | 12 minutes to read


Here is a List of essential SQL Server Interview Questions and Answers for Freshers and mid level of Experienced Professionals. All answers for these SQL Server questions are explained in a simple and easiest way. These basic, advanced and latest SQL Server questions will help you to clear your next Job interview.

SQL Server Interview Questions and Answers

These interview questions are targeted for SQL Server for developers and DBA. You must know the answers of these frequently asked SQL Server questions to clear the interview. This list includes questions based on joins, complex queries, performance tuning, cluster etc.

1. What are the ACID properties? Explain each of them.

ACID is a set of four properties. Let's understand each of them.

  • A - Atomicity ensures that each transaction is in a state of "all" or "nothing" means if one part of the transaction fails, the entire transaction fails and the database state is left unchanged. An atomic system must guarantee atomicity in every situation, including errors, crashes and power failures, .
  • C - Consistency confirms that any database transaction will bring the database from one valid state to another. Any data written to the database must be valid as per all defined rules, including cascades, constraints, triggers, and any combination thereof.
  • I - Isolation property ensures that the concurrent execution of transactions should not mix with each other and transactions result in a state that looks like transactions were processed serially, i.e. one after the other. Many data users can access the same data at the same time with lower isolation level but it increases the chances of concurrency effects that users might experience.
  • D - Durability ensures that once a database transaction has been committed, it will remain so, even in the event of power loss, crashes, or errors. In RDBMS, once a group of SQL statements execute, the results need to be stored permanently in the database even if the database crashes immediately thereafter. To defend against power loss or any database crash, all transactions and their effects must be recorded in a non-volatile memory.

2. What is UNION in SQL Server? How will you differentiate it from UNION ALL?

UNION merges the contents of two tables which are structurally compatible into a single combined table.
The difference is that UNION removes duplicate records whereas UNION ALL includes duplicate records.
UNION ALL has better performance then UNION, since UNION requires the server to do the extra work of eliminating any duplicates. So, in the cases where it is certain that there will not be any duplicates, or where having duplicates is not a problem, use of UNION ALL would be recommended for better performance.

3. What is the difference between the WHERE and HAVING clauses? What is Group by clause?

  • Where clause is used to filter the records from a result set. Filtering occurs before any grouping is made. It works with select clauses. it does not work with aggregate functions or GROUP BY statements.
    Select * from Employee Where Id > 10;
  • Having Clause is used to filter the records from groups. It works with group by clause and works on aggregate functions.
    Select Name, Salary from Employee   
    Group by Name, Salary   
    Having SUM(Salary)  > 10000; 
  • Group by clause is used to display the data in the form of identical groups. It is used with a SELECT Clause to group the result set by one or more columns.
    Select col1, col2, from table 
    group by col1, col2

4. What is an Index? Define Clustered and Non-Clustered index.

Index is an on-disk structure associated with a table or view that speeds the getting of rows from a database table or view. Index contains keys built from one or more column(s). These keys are stored in a special form of data structure known as B-tree that enables the SQL Server to find the rows associated with these keys very quickly. For more visit Index in SQL Server
SQL Server has 2 types of Indexes.
  • Clustered Index - When you create a table with a Primary key, SQL Server automatically creates the clustered index based on columns included in the primary key. Clustered indexes sort and store the data rows in the table or view as per their key values. These are the columns which are the part of the index definition. A data table has only one clustered index, because the data rows themselves can be sorted in only one order.
    A table with a clustered index is called a clustered table. Data rows of a table with no clustered index are stored in an unordered structure called heap. A table stores the data in sorted order only when it contains a clustered index. If you add a primary key constraint to an existing table that already contains a clustered index, SQL Server will enforce the primary key using a non-clustered index.
    For more visit Clustered Index.
  • Non clustered indexes have a structure separate from the data rows. A Non-clustered index includes the Non-clustered index key values and each key value entry has a pointer to the data row that contains the key value.
    In a Non-clustered index, the pointer from an index row to a data row is called a row locator. The structure of the row locator depends on whether the data pages are stored in a clustered table or a heap. If data pages are stored in a heap then row locator is a pointer to the row and if data pages are stored in a clustered table then row locator is the clustered index key. A table may have one or more non clustered index and each non clustered index may have one or more columns.
    For more visit Non Clustered Index.
For more about indexing questions visit SQL Server Indexes Questions.

5. How to delete Duplicate Records in SQL Server?

We can use CTE, Row_Number() with an over clause.

With CTE AS (
  select *,RN = ROW_NUMBER() over(partition by id Order by id) from Employee1

  delete from CTE where RN>1;
id - is the column by which you find duplicates.
OR you can also use below code if the table has an identity column to identify duplicate data.

FROM MyTable
FROM MyTable
GROUP BY DuplicateColumn1, DuplicateColumn2, DuplicateColumn3)

6. Explain Row_Number, RANK and DENSE_Rank in SQL Server.

  • Row_Number() is used to return a unique sequential number for each row starting from 1. If the partition clause is used with Row_Number then it reset sequential numbers for each partition. It does not skip or repeat the numbers in result.
  • RANK() is used to return a unique number for each distinct row starting from 1, within the partition if a partition clause is used. It starts at 1 in each partition. It sets the same rank for duplicate data and leaves the gaps in the rank sequence after duplicate values.
  • DENSE_RANK() has similar behavior like the RANK function but there is one difference that it does not leave the gaps in sequential rank after duplicate values.
    For example, consider the set {5, 5, 10, 15, 15, 20}. So here RANK() will return {1, 1, 3, 4, 4, 6} (note that the values 2 and 5 are skipped because of duplicate values assigned same rank and after duplication values there will be gap in rank), whereas DENSE_RANK() will return {1,1,2,3,3,4}.

USE [Practice]

/****** Object: Table [dbo].[Employee] Script Date: 31-05-2019 10:50:47 ******/


CREATE TABLE [dbo].[Employee] (
    [Id]     INT          NOT NULL,
    [Name]   VARCHAR (50) NULL,
    [Salary] VARCHAR (50) NULL

INSERT INTO [dbo].[Employee] ([Id], [Name], [Salary]) VALUES (1, N'bhanu',N'100')
INSERT INTO [dbo].[Employee] ([Id], [Name], [Salary]) VALUES (2, N'bhanu',N'200')
INSERT INTO [dbo].[Employee] ([Id], [Name], [Salary]) VALUES (3, N'Faizan',N'200')
INSERT INTO [dbo].[Employee] ([Id], [Name], [Salary]) VALUES (4, N'Faizan',N'200')
INSERT INTO [dbo].[Employee] ([Id], [Name], [Salary]) VALUES (5, N'Kap', N'300')
INSERT INTO [dbo].[Employee] ([Id], [Name], [Salary]) VALUES (6, N'Jap', N'100')
INSERT INTO [dbo].[Employee] ([Id], [Name], [Salary]) VALUES (7, N'Abhi', N'500')
INSERT INTO [dbo].[Employee] ([Id], [Name], [Salary]) VALUES (8, N'Shan', N'200')

;with CTE AS(select *, ROW_NUMBER() over(partition by Name,Salary order by Id) as RN from Employee)
select * from CTE
--select * from CTE where RN>1     -- here you can delete duplicate records where RN>1
-- delete from CTE where RN>1

select *, ROW_NUMBER() over(order by Id) as RN from Employee
select *, ROW_NUMBER() over(partition by Name,Salary order by Id) as RN from Employee

select *, RANK() over(order by Name) as rn from Employee
select *, RANK() over(partition by Name order by Name) as rn from Employee

select *, Dense_RANK() over(order by Name) as rn from Employee
select *, Dense_RANK() over(partition by Salary order by Name) as rn from Employee

-- find nth highest salary - always use dense rank as rank will skip some numbers.
-- so it will not give any result for those skipped numbers
select * from Employee
;with CTE1 as (select *, RANK() over (order by salary desc) as RN from Employee)
select top 1 * from CTE1 where RN=4   -- 4th Highest from highest to lowest

select * from Employee
;with CTE1 as (select *, DENSE_RANK() over (order by salary desc) as RN from Employee)
select top 1 * from CTE1 where RN=2  -- 2nd highest from
ROW_NUMBER, RANK, DENSE_RANK can be used with partition or without it but not without an over clause.

7. How to select top nth Records?

Let's take the example to select top 5th record from down

select top 1 id from (select top 5 id from employee1 order by id desc)sub order by id asc

8. Write a self join query with the following table structure.

Table select query is

    SELECT [id],[ename],[managerId] FROM [Practic].[dbo].[employee1]

    // Self Join Example: we will fetch emp name and manager name from the same table using self join.

    select e.ename, m.ename from employee1 e inner join employee1 m on e.managerId =

9. What is SQL Server Profiler?

SQL Profiler is an graphical user interface (GUI) tool that is used for tracing, recreating and troubleshooting the problems in SQL Server. It's used to identify slow executing queries and production problems by capturing the events. For example, you can identify slow executing stored procedures which are affecting performance in the production environment. SQL Profiler is used for the following type of activities.
  • Finding and diagnosing slow-running queries.
  • Correlating performance counters to diagnose problems.
  • Stepping through problem queries to find the cause of the problem.
  • Monitoring the performance of SQL Server to tune workloads
For more visit SQL Server Profiler.

10. Write a query to select all the Even and Odd number records from a table.

To select all even number records:

To select all odd number records:
Select * from table where id % 2 != 0

11. Why are stored procedures fast compared to running queries by c#?

Stored procedures are fast because these are in compile form, meaning no need to compile when we run it. Whenever we run some query by C# (ORM or then first the query gets compiled and execution plan is created but in case of stored procedures execution plan already exists as it was already created at the time of writing stored procedures.

12. What is Self Join and why is it required?

Self Join provides the capability of joining one table with itself. For example, you have one table 'employee' with three columns id, name, manager_id. Now you want to print the name of the employee and his manager in the same row.

WHERE e.manager_id = 

13. What is the difference between Truncate, Delete and Drop commands?

All these are the SQL commands used on the basis of different needs as below.

  • Truncate - It's Data Definition Language (DDL) command in SQL Server. That's why the Truncate operation can not be rolled back. It's used to delete the content of a table and free the space.
  • Delete - It's Data Manipulation Language (DML) command. That's why Delete operation can be rolled back. Delete command is used to delete the records from a table.
  • Drop - It's Data Definition Language (DDL) command. That's why Drop operation can not be rolled back. Drop command is used to remove the object from Database.

14. How can you improve Stored Procedure Performance?

You should focus on certain points for your Stored Procedure performance as below.

  • Use Proper indexing on tables.
  • Set NOCount ON|OFF - it will control some messages like - after running some update query you see messages - '0 rows affected'.
  • Use Select count(1) instead of count(*) for count function.
  • do not use prefix 'SP' while creating stored procedures as default system stored procedures also starts with prefix 'SP'.
  • Whenever it's required fetch data from the table with 'NOLOCK' keyword WITH(NOLOCK).
  • Set ANSI_Nulls ON|OFF- When it's ON it means a select statement will return zero results even if there are null values in the column. When It's OFF means select statement will return the corresponding rows with null values in columns. When you are setting it OFF means comparison operators do not follow ISO standards.
  • Set Quoted_Identifier ON|OFF - When it's ON means identifiers are delimited by double quotes and literals are delimited by single quotes. But when it's OFF means identifiers can not be delimited by quotation.

15. How to concatenate text from multiple rows into a single text string in SQL server? Consider the rows below.


Expected O/p from above 3 rows should be - Test1, Test2, Test3
You can achieve about output using below SQL query:

    DECLARE @Names VARCHAR(8000)
    SELECT @Names = COALESCE(@Names + ', ','') + Name
    FROM Common.Category
    Select @Names

    // If row contain null values then Coalesce can give wrong results so handle null case as below:
    DECLARE @Names VARCHAR(8000)
    SELECT @Names = COALESCE(@Names + ', ','') + Name
    FROM Common.Category Where Name IS NOT NULL
    Select @Names

    // OR
    DECLARE @Names VARCHAR(8000)
    SELECT @Names = COALESCE(@Names + ', ','') + ISNULL(Name, 'N/A')
    FROM Common.Category
    Select @Names

16. How to UPDATE from a SELECT in SQL Server?

Sometimes, We need to update table data from other tables data. In this case we prefer to use select with update command as below.

    UPDATE Emp
    FROM Employees Emp
    AddressList Address
    ON Emp.PersonId = Address.PersonId
For more visit Update from a Select command in SQL Server.

17. How to insert the result of a stored procedure into a temporary table?

18. How to convert rows to columns in SQL Server?

19. How to split a comma-separated value to columns in SQL Server?

20. How to check Query Execution Plan in SQL Server?

For more about execution plan visit Execution Plan in SQL Server.

21. What is the difference between Count(*) and Count(1)?

22. How to escape a single quote in SQL Server?

23. What is the difference between NOT IN vs NOT EXISTS?

24. How to return only the Date from a DateTime datatype in SQL Server?

25. How to get column values whose value starts with 'a' letter?

26. What is the use of CTE in SQL Server?

26. Explain the Magic Tables in SQL Server.

26. Differentiate Functions and Stored Procedures in SQL Server.

26. What is the OPTION clause in SQL Server.

26. Explain the use of Coalesce function in SQL Server.

For more visit Coalesce function in SQL Server.

26. How will you improve database performance in SQL Server.

For more visit Improve Performance of SQL Server Database and Improve Performance of SQL Server Database

Some General Interview Questions for SQL Server

1. How much will you rate yourself in SQL Server?

When you attend an interview, Interviewer may ask you to rate yourself in a specific Technology like SQL Server, So It's depend on your knowledge and work experience in SQL Server.

2. What challenges did you face while working on SQL Server?

This question may be specific to your technology and completely depends on your past work experience. So you need to just explain the challenges you faced related to SQL Server in your Project.

3. What was your role in the last Project related to SQL Server?

It's based on your role and responsibilities assigned to you and what functionality you implemented using SQL Server in your project. This question is generally asked in every interview.

4. How much experience do you have in SQL Server?

Here you can tell about your overall work experience on SQL Server.

5. Have you done any SQL Server Certification or Training?

It depends on the candidate whether you have done any SQL Server training or certification. Certifications or training are not essential but good to have.


We have covered some frequently asked SQL Server Interview Questions and Answers to help you for your Interview. All these Essential SQL Server Interview Questions are targeted for mid level of experienced Professionals and freshers.
While attending any SQL Server Interview if you face any difficulty to answer any question please write to us at Our IT Expert team will find the best answer and will update on the portal. In case we find any new SQL Server questions, we will update the same here.