What is different between nullif and isnull? [Resolved]

Dear Sudhakar,

NULLIF : It is a Function
1. Returns a NULL value if the two specified expressions are equal
2. Returns first expression, If the specified expressions are NOT equal
/*Both expressions(@A and @B) are Equal*/

Declare @A Int, @B Int

Select @A = 100, @B = 100

Select NULLIF(@B, @A) [Result]

Result 

--------- 

NULL



/*Both expressions(@A and @B) are NOT Equal*/



Declare @A Int, @B Int



Select @A = 100, @B = 101



Select NULLIF(@B, @A) [Result]

Result 

--------- 

101 

ISNULL : It is a Function

- Returns second Expression, If the first one is NULL
- Returns first expression, If the first one is NOT NULL

Declare @A Int

Select @A = 100

Select ISNULL(@A,0) [Result]



Result 

--------- 

100

Happy Coding
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Explanation of NULLIF
Syntax:
NULLIF ( expression , expression )

Returns a null value if the two specified expressions are equal. NULLIF returns the first expression if the two expressions are not equal. If the expressions are equal, NULLIF returns a null value of the type of the first expression. NULLIF is equivalent to a searched CASE function in which the two expressions are equal and the resulting expression is NULL.

NULLIF() Returns a null value if the two specified expressions are equal. If the Two expressions are not equal then it will return the first expression's value. Whether the returned value is NULL or NOT NULL, NULLIF() will return the same data type as the first expression


Explanation of ISNULL
Syntax:
ISNULL ( check_expression , replacement_value )

Replaces NULL with the specified replacement value. The value of check_expression is returned if it is not NULL; otherwise, replacement_value is returned after it is implicitly converted to the type of check_expression, if the types are different.

ISNULL() Replaces the NULL value with the specified expression value.

Observation:
Interesting observation is NULLIF returns null if it comparison is successful, where as ISNULL returns not null if its comparison is successful. In one way they are opposite to each other.


Examples:

Example 1 :

Select NULLIF(100,50*2)  'NULLIFValue' 

OutPut :

NULL

Example 2 :

Select NULLIF(2*2,2*7)  'NULLIFValue' 



OutPut :

4

Example 3 :

Select NULLIF(20-2,19)  'NULLIFValue' 



OutPut :

NULLIFValue

18

ISNULL Examples
Example 1 :

Select ISNULL(null,12) 'ISNULL VALUE'

OutPut :

ISNULLVALUE

12

Simply ISNULL replaces first argument with second argument if first argument value is NULL; otherwise first argument value.
NULLIF() return NULL if first & second arguments are equal; otherwise first argument value

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Sudhakar_A, if this helps please login to Mark As Answer. | Alert Moderator

COALESCE( exp1, exp2, ....) is also same as ISNULL() function in SQL Server..
To replace NULLs with other values we supposed to use ISNULL()/COALESCE() functions...
The difference between ISNULL() and COALESCE() is that COALESCE() will take more than two arguments... where as Isnull() will take two params and returns value

DECLARE @A int = NULL, @B INT = NULL, @C INT = 10



SELECT  ISNULL( @A, @B) -- returns NULL because of second argument value too NULL

SELECT  ISNULL(ISNULL( @A, @B) , @c) -- returns 10 

 equivalent COALESCE() is as follows:

SELECT COALESCE( @A, @B, @C) -- Returns 10

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Sudhakar_A, if this helps please login to Mark As Answer. | Alert Moderator

Hi , here is a video which explains Differences between ISNULL and NULLIF in detail.

https://www.youtube.com/watch?v=waiKG7pPqHg

I hope this helps you in clarifying your doubt!!

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