SQL can I have a "conditionally unique" constraint on a table?

Question

I've had this come up a couple times in my career, and none of my local peers seems to be able to answer it. Say I have a table that has a "Description" field which is a candidate key, except that sometimes a user will stop halfway through the process. So for maybe 25% of the records this value is null, but for all that are not NULL, it must be unique.

Another example might be a table which must maintain multiple "versions" of a record, and a bit value indicates which one is the "active" one. So the "candidate key" is always populated, but there may be three versions that are identical (with 0 in the active bit) and only one that is active (1 in the active bit).

I have alternate methods to solve these problems (in the first case, enforce the rule code, either in the stored procedure or business layer, and in the second, populate an archive table with a trigger and UNION the tables when I need a history). I don't want alternatives (unless there are demonstrably better solutions), I'm just wondering if any flavor of SQL can express "conditional uniqueness" in this way. I'm using MS SQL, so if there's a way to do it in that, great. I'm mostly just academically interested in the problem.

Answer 1

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

Answer 2

In the case of descriptions which are not yet completed, I wouldn't have those in the same table as the finalized descriptions. The final table would then have a unique index or primary key on the description.

In the case of the active/inactive, again I might have separate tables as you did with an "archive" or "history" table, but another possible way to do it in MS SQL Server at least is through the use of an indexed view:

CREATE TABLE Test_Conditionally_Unique
(
    my_id   INT NOT NULL,
    active  BIT NOT NULL DEFAULT 0
)
GO
CREATE VIEW dbo.Test_Conditionally_Unique_View
WITH SCHEMABINDING
AS
    SELECT
        my_id
    FROM
        dbo.Test_Conditionally_Unique
    WHERE
        active = 1
GO
CREATE UNIQUE CLUSTERED INDEX IDX1 ON Test_Conditionally_Unique_View (my_id)
GO

INSERT INTO dbo.Test_Conditionally_Unique (my_id, active)
VALUES (1, 0)
INSERT INTO dbo.Test_Conditionally_Unique (my_id, active)
VALUES (1, 0)
INSERT INTO dbo.Test_Conditionally_Unique (my_id, active)
VALUES (1, 0)
INSERT INTO dbo.Test_Conditionally_Unique (my_id, active)
VALUES (1, 1)
INSERT INTO dbo.Test_Conditionally_Unique (my_id, active)
VALUES (2, 0)
INSERT INTO dbo.Test_Conditionally_Unique (my_id, active)
VALUES (2, 1)
INSERT INTO dbo.Test_Conditionally_Unique (my_id, active)
VALUES (2, 1)    -- This insert will fail

You could use this same method for the NULL/Valued descriptions as well.