Is there a way to add a unique constraint to a column that will enforce uniqueness, but allow multiple nulls? From reading books online, it looks like you can do it. Although, when I try to create it it fails because of duplicate nulls.<BR><BR>From books online....<BR><BR>When a UNIQUE constraint is added to an existing column or columns in the table, Microsoft® SQL Server™ by default checks the existing data in the columns to ensure all values, except NULL, are unique. If a UNIQUE constraint is added to a column that has duplicated values, SQL Server returns an error and does not add the constraint.<BR><BR>Here is what I&#039ve been typing....<BR><BR><BR>ALTER TABLE device<BR>ADD CONSTRAINT u_device_control_number<BR>UNIQUE (device_control_number)<BR>GO<BR>