On Apr 11, 2008, at 3:47 PM, Phillip Gawlowski wrote:
> Most database systems can handle that, be defining a UNIQUE constraint
> on columns.
>
> However, that means that A = 1 and B = 2 won't work.

Sure it will- you define the unique index to span the two columns, and  
then only non-unoque combinations of those columns will fail.

Dave