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.
Following is good example of NULLIF and CASE from BOL:
USE AdventureWorks;
GO
SELECT ProductID, MakeFlag, FinishedGoodsFlag,
NULLIF(MakeFlag,FinishedGoodsFlag)AS 'Null if Equal'
FROM Production.Product
WHERE ProductID < 10;
GO
SELECT ProductID, MakeFlag, FinishedGoodsFlag,'Null if Equal' =
CASE
WHEN MakeFlag = FinishedGoodsFlag THEN NULL
ELSE MakeFlag
END
FROM Production.Product
WHERE ProductID < 10;
GO
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.
Following is good example of ISNULL from BOL:
USE AdventureWorks;
GO
SELECT AVG(ISNULL(Weight, 50))
FROM Production.Product;
GO
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.
Puzzle:
How to create infinite loop using NULLIF and ISNULL? If this is even possible?
Reference : Pinal Dave (http://blog.SQLAuthority.com)
pinaldave
Thanks, Nice tip
Maybe this ?
IsNull(null, nullif(1, 1)), but I guess it would just return null :P
SET ANSI_NULLS OFF
DECLARE @test AS INT
SET @test = 1
WHILE ISNULL( NULLIF(@test, @test), @test ) = @test
BEGIN
PRINT ‘HI’
BREAK
END
Making @test = NULL would still make it print ‘HI’
Hello The Waterproof SQL Champ good work,
I was expecting it to be done without using WHILE but I did not make that clear in question so it works.
Good Work. I modified your code to show how the infinite code can be generated. However following script will run till 10 iteration and will stop.
SET ANSI_NULLS OFF
DECLARE @test AS INT
SET @test = 1
WHILE ISNULL( NULLIF(@test, @test), @test ) = @test
BEGIN
PRINT ‘HI’
SET @test = @test + 1
IF @test > 10
BREAK
END
GO
Warning : Do not run this code on production machine. Make sure you test it. The reason it is published here, so user who have similar problem can prevent infinite loop from happening.
I have the following code that uses the IsNull function.
declare @Test bit;
set @Test = ‘false’;
select ( Case When IsNull(@Test, ”) = ”
Then ‘IS NULL’
Else ‘NOT NULL’
End);
I would expect ‘NOT NULL’ to be returned as @Test is set to a value ‘false’. However ‘IS NULL’ is output instead.
Does SQL Server treat NULL the same as ‘false’?
Yasar,
You should rephrase this to something like this
declare @Test bit
set @Test = NULL
/*
Change to
set @Test = 0 or
set @Test = 1
keeping every other SET line remarked to check different
behaviours.
*/
select Case IsNull(@Test, 0) —
when 0 Then ‘IS NULL’
else ‘NOT NULL’
End Result
That will work just fine.
declare @Test tinyint
set @Test = NULL
/*
Change to
set @Test = 0 or
set @Test = 1
keeping every other SET line remarked to check different
behaviours.
*/
select Case IsNull(@Test, 2) —
when 2 Then ‘IS NULL’
else ‘NOT NULL’
End Result
That should do it.
I have a serious of bit values. I want to display column if bit value is 1 and hide column if bit value is 0. How can I achieve this. Here is what I have so far.
ALTER PROCEDURE [dbo].[sp_datadump_selectfromcheckbox]
@project_ID int,
@Title_isChecked bit
AS
BEGIN
SELECT PRSDATA.Anomaly_ID,
CASE WHEN @Title_isChecked =’1′ THEN (Select PRSDATA.Title) END ‘ ‘
from PRSDATA,PRSDATA1, PRSDATA2, PRSDATA3, PRSDATA4, PRSDATA5, PRSDATA6, PRSDATA7 , PRSDATA8
WHERE PRSDATA.anomaly_id = PRSDATA1.anomaly_id and
PRSDATA.anomaly_id = PRSDATA2.anomaly_id and
PRSDATA.anomaly_id = PRSDATA3.anomaly_id and
PRSDATA.anomaly_id = PRSDATA4.anomaly_id and
PRSDATA.anomaly_id = PRSDATA5.anomaly_id and
PRSDATA.anomaly_id = PRSDATA6.anomaly_id and
PRSDATA.anomaly_id = PRSDATA7.anomaly_id and
PRSDATA.anomaly_id = PRSDATA8.anomaly_id and
PRSDATA.Main_Project_Affected = @project_ID
END
if (@@error = 0)
begin
RETURN 0
end
else
begin
RETURN 1
end
[...] SQL SERVER – Explanation and Comparison of NULLIF and ISNULL [...]
Now, here’s a fun one… if you don’t want to set ansi_nulls, you can use the isnull()/nullif() combo when comparing two values in a table.
The trick is in this line: isnull(nullif(val1, val2), nullif(val2, val1)) is null
————————————–
————————————–
if object_id(’tempdb..#test’) is not null drop table #test
create table #test (
val1 varchar(50),
val2 varchar(50)
)
insert into #test
select ‘Bill’, ‘Bill’
union all
select null, null
union all
select ‘Bill’, null
union all
select ‘Bill’, ‘Gates’
union all
select null, ‘Gates’
select
val1,
val2,
case
when val1 = val2 then ‘(wrong) equal’
else ”
end as WrongEquals,
case
when (val1 val2) then ‘(wrong) NOT equal’
else ”
end as WrongNotEquals,
NULLIF(val1, val2) as NullIf1_2,
NULLIF(val2, val1) as NullIf2_1,
case
when isnull(nullif(val1, val2), nullif(val2, val1)) is null then ‘eqivalent!’
else ”
end as OddlyCorrect_Equivalent,
case
when isnull(nullif(val1, val2), nullif(val2, val1)) is not null then ‘NOT eqivalent!’
else ”
end as OddlyCorrect_NotEquivalent
from
#test
————————————–
————————————–
Enjoy wrapping your mind around that.