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null value in unique constraint

  • August 29, 2009 at 2:20 am

    #134644

    Hi,I have a question that I can't find the answer anywhere...I know that in sql2005 the unique constraint can keep just one null record,but I don't know that why it can keep one null and the usage of this null record in sql2005??...if someone know the answer please tell it....thanks in advance

  • August 29, 2009 at 7:12 am

    #1046475

    There's no real use. It's just that within unique indexes is the only place where nulls are considered the same. Unique indexes and unique constraints only allow a single null. Irritating, but that's how it it.

    Gail Shaw
    Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
    SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability

    We walk in the dark places no others will enter
    We stand on the bridge and no one may pass

  • August 30, 2009 at 8:58 pm

    #1046702

    GilaMonster (8/29/2009)

    It's just that within unique indexes is the only place where nulls are considered the same.

    NULLs are also considered 'the same' by GROUP BY, DISTINCT, UNION, EXCEPT, INTERSECT...and probably others 😛

    Paul White
    All articles available on SQL.kiwi
    @SQL_Kiwi

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