Unique Constraint with Multiple NULLS

What is the SQL-92 standard for allowing multiple NULL values within a unique constraint?

Since NULL is the absence of value I would think that multiple NULLS within a unique constraint would be allowed, however the RDBMS disagrees. Shouldn't a constraint only be concerned with values, not non-values?

Besides a trigger is there any other reasonable way to enforce this behavior if the RDBMS does not support it?

Thanks,

Adam Received on Tue Dec 30 2003 - 22:21:54 CST