Unique Constraint, excluding NULL values [duplicat

问题:

Possible Duplicate:
The proper way to implement unique constraint that allows multiple NULL values in SQL Server

I have a column in my table where all the values must either be unique, or NULL.

I tried adding a Unique Key to the table for this column, but this seems to mean I can only have 1 NULL value?

How can I set a constraint so that all the values are unique, unless they are NULL?

回答1:

SQL Server 2008 has filtered indexes that allow this but they are not available in 2005. In SQL Server 2005 you can create an indexed view with definition

CREATE VIEW dbo.Foo
WITH SCHEMABINDING
AS
SELECT bar
FROM dbo.baz
WHERE bar IS NOT NULL

Then create a unique clustered index on that.

CREATE UNIQUE CLUSTERED INDEX ix ON dbo.Foo(bar)