Need Syntax Help Writing Constraint/Rule

[avanderlaan]

I have a table with the following columns
-- workOrderId int,
-- serialNumber char(20),
-- other columns...

I would like to impose a constraint on the above-named columns such that either
1) the combination workOrderId/serialNumber is UNIQUE, -or-
2) the serialNumber string must be empty.

Is this possible? Thanx in advance.