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Difference between null and coalesce

ISNULL

  1. ISNULL function is used to replace the NULL value with specified value.
  2. It contains only two arguments.
  3. Same data type not compulsory.

Example -1:

SELECT ISNULL(NULL,'Raki') AS [ISNULL]

Output: Raki

Example -2:

DECLARE @name VARCHAR(10)
DECLARE @marks INT =500
SELECT ISNULL(@name,@marks) AS [ISNULL]

Output: 500

Example-3:

SELECT ISNULL(NULL,NULL,'Raki') AS [ISNULL]     

Output:

  Msg 174, Level 15, State 1,
  The isnull function requires 2 argument(s).

COALESCE

  1. Coalesce function is returns first non null value among arguments.  
  2. It contains multiple arguments.
  3. Same data type compulsory for arguments or precedence data type order should follow.


Example-1:

SELECT COALESCE(NULL,NULL,'Raki') as [COALESCE]

Output: Raki

Example-2:

DECLARE @name VARCHAR(5)='Raki'
DECLARE @marks INT =500
SELECT COALESCE(@name,@marks) as [COALESCE]

Output:  
Msg 245, Level 16, State 1, Line 3
Conversion failed when converting the varchar value 'Raki' to data type int.


         



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