What's the cleanest way to constrain a SQL table to allow it to have no more than one row?

[This related question](https://stackoverflow.com/questions/1208344/sql-table-with-a-single-row) 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