07/Jul/2020 | 10 minutes to read
QFLES is listing some Essential SQL Server Interview Questions and Answers for Freshers and mid level of Experienced Professionals prepared by Industry Experts. All answers for these SQL Server interview questions are given based on standard documents and tried to explain in simple and easiest way.
1. What are the ACID properties? Or list and explain each of the ACID properties that collectively guarantee that database transactions are processed reliably.
ACID (A - Atomicity, C - Consistency, I - Isolation, D - Durability) is a set of properties that guarantee that database transactions are processed reliably. They are defined as follows:
2. What is UNION in SQL Server? What is the difference between UNION and UNION ALL?
UNIONit merges the contents of two tables which are structurally compatible into a single combined table.
DifferenceThe difference between UNION and UNION ALL is that UNION will omit duplicate records whereas UNION ALL will include duplicate records.
3. What is the difference between the WHERE and HAVING clauses? What is Group by clause?
Where clause:Where clause is used to filter the records from a result set. Filtering occurs before any grouping made. it works with select clause. it does not work with aggregate functions or group by.
Select * from Employee Where Id > 10;
Having ClauseHaving 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 clauseGroup by clause is used to display the data in the form of identical groups. it is used with select clause to group the result set by one or more columns.
Select col1, col2, from table group by col1, col2
4. What is Index? Define Clustered and Non-Clustered index.
IndexIndex is an on disk structure associated with table or view that is built from one or more columns in table or view. An Index speeds the retrieval of data from table or view. Index contains key built from one or more column(s). These keys are stored in the form of B-tree data structure that enables the SQL Server to find the rows associated with these keys in a quick and efficient way.
Clustered IndexWhen you create a table with Primary key, SQL Server automatically create the clustered index based on columns included in primary key. Clustered indexes sort and store the data rows in the table or view based on their key values. These are the columns included in the index definition. There can be only one clustered index per table, because the data rows themselves can be sorted in only one order.
Non Clustered IndexNon clustered index also speeds the retrieval of data from the tables but it sorts and stores the data separately from the data rows in the table. It is a copy of selected columns of data from a table with the links to the associated table. So Non-clustered indexes have a structure separate from the data rows. A Non-clustered index contains the Non-clustered index key values and each key value entry has a pointer to the data row that contains the key value.
5. How to delete Duplicate Records in SQL Server?
We can use CTE, Row_Number() with 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 column by which u find duplicates OR you can also use below code if table has identity column to identify duplicate data.
DELETE FROM MyTable WHERE ID NOT IN ( SELECT MAX(ID) FROM MyTable GROUP BY DuplicateColumn1, DuplicateColumn2, DuplicateColumn3)
6. Explain Row_Number, RANK and DENSE_Rank in SQL Server.
Row_Number()It's used to return unique sequential number for each row starting from 1. if partition clause is used with Row_Number then it reset sequential number for each partition. It does not skip or repeat the numbers in result.
RANK()It's used to return a unique number for each distinct row starting from 1, within the partition if partition clause is used. It starts at 1 in each partition. It sets same rank for duplicate data and leaves the gaps in the rank sequence after duplicate values.
DENSE_RANK()It has similar behavior like RANK function but there is one difference that it does not leave the gaps in sequential rank after duplicate values.
USE [Practice] GO /****** Object: Table [dbo].[Employee] Script Date: 31-05-2019 10:50:47 ******/ SET ANSI_NULLS ON GO SET QUOTED_IDENTIFIER ON GO 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) asRN 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 fromEmployee 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 fromEmployee -- 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 fromEmployee) select top 1 * from CTE1 where RN=2 -- 2nd highest fromROW_NUMBER, RANK, DENSE_RANK can be used with partition or without it but not without 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 self join query with 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 same table using self join.
select e.ename, m.ename from employee1 e inner join employee1 m on e.managerId = m.id
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 monitor a production environment to see which stored procedures are affecting performance by executing too slowly.
SQL Profiler is used for following type of activities.
10. Write a query to select all the Even and Odd number records from a table.
To select all even number records:
SELECT * FROM TABLE WHERE ID % 2 = 0To select all odd number records:
Select * from table where id % 2 != 0
11. Why stored procedures are fast as compare to run query by c#?
Stored procedures are fast because these are in compile form means no need to compile when we run it. Whenever we run some query by c# (ORM or ado.net) then first query get compiled then create execution plan but in case of store procedures execution plan does not get created again as it's already created when we write store procedures.
12. What is Self Join and why is it required?
Self Join provide 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 employee and his manager in same row.
SELECT e.name EMPLOYEE, m.name MANAGER FROM EMPLOYEE e, EMPLOYEE m WHERE e.manager_id = m.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.
14. How can you improve Stored Procedure Performance?
You should focus on certain points for your Stored Procedure performance as below.
Some General Interview Questions for SQL Server:
1. How much will you rate your self in SQL Server?
When you attend an interview, Interviewer may ask you to rate your self in 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 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's depend on candidate like you have done any SQL Server training or certification. Certifications or trainings are not essential but good to have.
Conclusion: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.