Constrain a table to have only one row

Posted by finnw on Stack Overflow See other posts from Stack Overflow or by finnw
Published on 2010-05-18T11:49:15Z Indexed on 2010/05/18 12:00 UTC
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What's the cleanest way to constrain a SQL table to allow it to have no more than one row?

This related question discusses why such a table might exist, but not how the constraint should be implemented.

So far I have only found hacks involving a unique key column that is constrained to have a specific value, e.g. ALWAYS_0 TINYINT NOT NULL PRIMARY KEY DEFAULT (0) CONSTRAINT CHECK_ALWAYS_0 CHECK (ALWAYS_0 = 0). I am guessing there is probably a cleaner way to do it.

The ideal solution would be portable SQL, but a solution specific to MS SQL Server or postgres would also be useful

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