SQL can I have a “conditionally unique” constraint on a table?
If you are using SQL Server 2008 a Index filter would maybe your solution: http://msdn.microsoft.com/en-us/library/ms188783.aspx This is how I enforce a Unique Index with multiple NULL values CREATE UNIQUE INDEX [IDX_Blah] ON [tblBlah] ([MyCol]) WHERE [MyCol] IS NOT NULL