What is the use of allowing one null in Unique key

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Hi ,

I want to know what is the use of allowing only one null value in Unique Key column.

thanks.

Posted 19-Oct-12 7:30am

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Sandeep Mewara · Accepted Answer · 2012-10-19T07:47:00

Solution 1

Logically, any key which is allowed to contain non duplicate (unique) values is a unique key, NULL is a permissible value in sql server , so it can have NULL for a single time just like any other value.

But as per standards, there is no such rule that only ONE NULL is allowed based on standards.
Refer:
http://en.wikipedia.org/wiki/Unique_key[^]
http://connect.microsoft.com/SQLServer/feedback/details/299229/change-unique-constraint-to-allow-multiple-null-values[^]

Posted 19-Oct-12 7:47am

Sandeep Mewara

Comments

fjdiewornncalwe 19-Oct-12 14:42pm

+5. Excellent.
As an additional note. The one NULL value will only be valid if the column allows NULLs, so this UNIQUE constraint is somewhat tied to that property as well.

Sandeep Mewara 20-Oct-12 0:45am

Thanks. And yes, your additional comments are to be kept in mind.

Sergey Alexandrovich Kryukov 19-Oct-12 15:26pm

5ed.
--SA

Sandeep Mewara 20-Oct-12 0:45am

Thanks SA.

Maciej Los 21-Oct-12 5:48am

Good answer, +5!

Sandeep Mewara 21-Oct-12 5:48am

Thanks Mac. :)