Question

I want to have a unique constraint on a column which I am going to populate with GUIDs. However, my data contains null values for this columns. How do I create the constraint that allows multiple null values?

Here’s an example scenario. Consider this schema:

CREATE TABLE People (
  Id INT CONSTRAINT PK_MyTable PRIMARY KEY IDENTITY,
  Name NVARCHAR(250) NOT NULL,
  LibraryCardId UNIQUEIDENTIFIER NULL,
  CONSTRAINT UQ_People_LibraryCardId UNIQUE (LibraryCardId)
)

Then see this code for what I’m trying to achieve:

-- This works fine:
INSERT INTO People (Name, LibraryCardId) 
 VALUES ('John Doe', 'AAAAAAAA-AAAA-AAAA-AAAA-AAAAAAAAAAAA');

-- This also works fine, obviously:
INSERT INTO People (Name, LibraryCardId) 
VALUES ('Marie Doe', 'BBBBBBBB-BBBB-BBBB-BBBB-BBBBBBBBBBBB');

-- This would *correctly* fail:
--INSERT INTO People (Name, LibraryCardId) 
--VALUES ('John Doe the Second', 'AAAAAAAA-AAAA-AAAA-AAAA-AAAAAAAAAAAA');

-- This works fine this one first time:
INSERT INTO People (Name, LibraryCardId) 
VALUES ('Richard Roe', NULL);

-- THE PROBLEM: This fails even though I'd like to be able to do this:
INSERT INTO People (Name, LibraryCardId) 
VALUES ('Marcus Roe', NULL);

The final statement fails with a message:

Violation of UNIQUE KEY constraint ‘UQ_People_LibraryCardId’. Cannot insert duplicate key in object ‘dbo.People’.

How can I change my schema and/or uniqueness constraint so that it allows multiple NULL values, while still checking for uniqueness on actual data?

Answer 1

SQL Server 2008 +

You can create a unique index that accept multiple NULLs with a WHERE clause. See this answer.

Prior to SQL Server 2008

You cannot create a UNIQUE constraint and allow NULLs. You need to set a default value of NEWID().

Update the existing values to NEWID() where NULL before creating the UNIQUE constraint.