How to create a unique index on a NULL column?

Question

I am using SQL Server 2005. I want to constrain the values in a column to be unique, while allowing NULLS.

My current solution involves a unique index on a view like so:

CREATE VIEW vw_unq WITH SCHEMABINDING AS
    SELECT Column1
      FROM MyTable
     WHERE Column1 IS NOT NULL

CREATE UNIQUE CLUSTERED INDEX unq_idx ON vw_unq (Column1)

Any better ideas?

Answer 1

Pretty sure you can't do that, as it violates the purpose of uniques.

However, this person seems to have a decent work around: http://sqlservercodebook.blogspot.com/2008/04/multiple-null-values-in-unique-index-in.html

Answer 2

The calculated column trick is widely known as a "nullbuster"; my notes credit Steve Kass:

CREATE TABLE dupNulls (
pk int identity(1,1) primary key,
X  int NULL,
nullbuster as (case when X is null then pk else 0 end),
CONSTRAINT dupNulls_uqX UNIQUE (X,nullbuster)
)

Answer 3

Using SQL Server 2008, you can create a filtered index: http://msdn.microsoft.com/en-us/library/cc280372.aspx. (I see Simon added this as a comment, but thought it deserved it's own answer as the comment is easily missed)

Another option is a trigger to check uniqueness, but this could affect performance.