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Unique Key

Author

Topic

singh_nirajan
Starting Member

2 Posts

Posted - 2008-07-07 : 09:22:38

Why unique key accept only one Null value?
Answer is another null is duplicate of existing null. I think this not correct answer. One NULL is never equal to another NULL.

Declare @OneNull varchar(5), @anotherNull varchar(5)

Set @OneNull = NULL
Set @anotherNull = NULL

If @OneNull = @anotherNull
Print 'True'
Else
Print 'False'

------------
Result is always : False

any one please explain the logic behind unique key accepting only one Null.

Nirajan Singh

Transact Charlie
Master Smack Fu Yak Hacker

3451 Posts

Posted - 2008-07-07 : 10:55:19

"One NULL is never equal to another NULL"

A NULL *could be* equal to another NULL. They both represent missing or incomplete data so they could be the same.

And your logic isn't complete: the result isn't false it is in fact NULL


DECLARE @a INT, @b INT

SELECT @a = NULL, @b = NULL

IF @a = @b PRINT 'EQUALS'
IF @a <> @b PRINT 'NOT EQUAL'

doesn't print anything.

Check out NULL in wikipedia -- has a good triple state logic overview.

-------------
Charlie

Michael Valentine Jones
Yak DBA Kernel (pronounced Colonel)

7020 Posts

Posted - 2008-07-07 : 12:22:31

quote:
Originally posted by singh_nirajan
...any one please explain the logic behind unique key accepting only one Null...

The logic behind it doesn’t really matter; it’s just the way SQL Server works.

If you really want it explained, contact Microsoft.

CODO ERGO SUM

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