Im trying to include validation for a field in one table to not allow an entry if that entry does not appear in another table.

EG: When entering in a serial number in a cost table, it will not allow the row to be saved, if that serial number does not appear in the revenue table.

EG: (Validation rule for the cost table's serial num field)...
[Table Cost].[Serial Num] = [Table Revenue].[Serial Num]

Can you do this??? Can I reference another table in a validation rule???