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Why ' set ANSI_NULL ON ' while writing SP


-- Create table t1 and insert values.
CREATE TABLE t1 (a INT NULL)
INSERT INTO t1 values (NULL)
INSERT INTO t1 values (0)
INSERT INTO t1 values (1)
GO


When SET ANSI_NULLS is ON, all comparisons against a null value evaluate to UNKNOWN. 


-- SET ANSI_NULLS to ON and test.
SET ANSI_NULLS ON
GO


DECLARE @varname int =  NULL
SELECT * FROM t1  WHERE a = @varname  -- No output
SELECT * FROM t1  WHERE a <> @varname -- No output
SELECT * FROM t1  WHERE a IS NULL     -- NULL 
GO


When SET ANSI_NULLS is OFF, comparisons of all data against a null value evaluate to TRUE if the data value is NULL


-- SET ANSI_NULLS to OFF and test.
SET ANSI_NULLS OFF 
GO


DECLARE @varname int =  NULL
SELECT * FROM t1  WHERE a = @varname  -- NULL
SELECT * FROM t1  WHERE a <> @varname -- 0 , 1
SELECT * FROM t1  WHERE a IS NULL     -- NULL 
GO


-- Drop table t1.
DROP TABLE t1
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