How to create fast database queries

Archive for July, 2013

10 things in SQL Server (which don’t work as expected)

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10. Unique NULL


When declaring UNIQUE constraints on columns, SQL Server treats all NULL values as unique. This means you can't have more than one NULL value in a unique column.

This behavior violates the standard and is specific to SQL Server.

To work around this, implement the constraint by declaring a UNIQUE index which would exclude NULL values:

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Written by Quassnoi

July 19th, 2013 at 11:00 pm

Posted in SQL Server