SQL SERVER Interview Questions & Answers – SET 2 (40 Questions & Answers)

Download – SQL SERVER Interview Questions with Answers – Set 2 [40 Questions&Answers]

Question1. We have a query which is running fine in development but facing performance issues at production. You got the execution plan from production DBA. Now you need to compare with the development execution plan. What is your approach? / Poor query performance in Prod.

Answer – This is an open ended question; There could be possible reasons for this issue. Some of them are given below-

1. Fragmentation could be one of the issue.
2. Check statistics are updated or not.
3. Check What other processes are running on production server.
4. Check if query is returning multiple query plans; if yes then there could be two reasons – Parameter sniffing or invalid plans due to set options
5. Check which indexes are getting used.
6. Examine the execution plan to find out the Red Flags. You have to understand what is going on bad and then figure out the alternatives.

Question 2. What is lock Escalation

Answer – Lock escalation is the process of converting many fine-grain locks into fewer coarse-grain locks, reducing system overhead while increasing the probability of concurrency contention. Every lock takes some memory space – 128 bytes on 64 bit OS and 64 bytes on 32 bit OS. And memory is not the free resource.

So if we have a table with billions of rows and we doing lot of operations on that table, SQL Server starts to use the process that called “Lock Escalation” i.e. instead of keeping locks on every row SQL Server tries to escalate them to the higher (object) level. As soon as you have more than 5.000 locks on one level in your locking hierarchy, SQL Server escalates these many fine-granularity locks into a simple coarse-granularity lock.

By default SQL Server always escalates to the table level. You can control lock escalation using below code.

Note lock goes from top to bottom ( DB -> Table -> Page -> Row )

--Handle Lock Escalation

ALTER TABLE Area
SET
(
LOCK_ESCALATION = AUTO -- or TABLE or DISABLE
)
GO

--

Notes – You can disable Lock Escalation for time being, but you got to be very careful with this. Yo can use loop to perform DELETE/UPDATE statements, so that you can prevent Lock Escalations. With this approach huge transactions will be divided into multiple smaller ones, which will also help you with Auto Growth issues that you maybe have with your transaction log.

Question 3. What is a Race Condition

Answer – A race condition is when two or more programs (or independent parts of a single program) all try to acquire some resource at the same time, resulting in an incorrect answer or conflict.

Question 4. Difference between ISNULL and COAELSCE

Answer –

Question 5. What is the difference between CROSS APPLY & OUTER APPLY IN T-SQL ?

Answer – The APPLY operator comes in two flavours, CROSS APPLY and OUTER APPLY. It is useful for joining two SQL tables or XML expressions. CROSS APPLY is equivalent to an INNER JOIN expression and OUTER APPLY is equivalent to a LEFT OUTER JOIN expression.

CREATE TABLE EmpApply
(
	 EmpId INT
	,EmpName VARCHAR(100)
	,DeptID INT
)
GO

INSERT INTO EmpApply VALUES
(1,'Rohit',1),
(2,'Rahul',2),
(3,'Isha',1),
(4,'Disha',NULL)

CREATE TABLE DeptApply
(
	 DeptID INT
	,Name VARCHAR(100)
)
GO

INSERT INTO DeptApply VALUES
(1,'IT'),
(2,'Finance')

SELECT * FROM EmpApply
CROSS APPLY ( SELECT * FROM DeptApply WHERE DeptApply.DeptID = EmpApply.DeptID )ax

--

Output of the above query is

--

SELECT * FROM EmpApply
OUTER APPLY ( SELECT * FROM DeptApply WHERE DeptApply.DeptID = EmpApply.DeptID ) ax

--

Output of the above query is

Question 6. What is stuff function? Difference between stuff and replace?

Answer- REPLACE – Replaces all occurrences of a specified string value with another string value.

 -----------------Syntax -------------------
REPLACE ( string_expression , string_pattern , string_replacement )

-----------------Example------------------- 

DECLARE @Text1 AS VARCHAR(100) = 'Pawan - A SQL Dev'
SELECT REPLACE(@Text1,'SQL','MSBI')

Output of the above query is

STUFF function is used to overwrite existing characters.

 -----------------Syntax -------------------
STUFF ( character_expression , start , length , replaceWith_expression )

-----------------Example------------------- 

DECLARE @Text AS VARCHAR(100) = 'Pawan - A SQL Dev'
SELECT STUFF(@Text,2,5,'NEW')

Output of the above query is

Question 7. How to change the port number for SQL Server? Default port no of SQL SERVER

Answer- Default PORT Number of SQL Server is 1433. You can view the port number under configuration Manager.

URL – https://msdn.microsoft.com/en-us/library/ms177440.aspx

--

CREATE TABLE testScan
(
	 ID INT IDENTITY(1,1) PRIMARY KEY
	,Name VARCHAR(10)
)
GO

INSERT INTO testScan(Name)
VALUES
('Isha'),
('Seema'),
('Ziva'),
('Sharlee')

SELECT * FROM testScan

--

--

--**********OLD PROC******************

CREATE PROC Area
(
	@ToPoint VARCHAR(20)
)
AS
	SELECT ID , FromPoint , ToPoint , Distance FROM Area
	WHERE ToPoint = @ToPoint



--**********NEW PROC with Local Variables******************


CREATE PROC Area
(
	@ToPoint VARCHAR(20)
)
AS
	DECLARE @tP AS VARCHAR(20) = @ToPoint

	SELECT ID , FromPoint , ToPoint , Distance FROM Area
	WHERE ToPoint = @tP


--