table ART contains all the items sold by a warehouse.

COD_ART is the record field that contains the item code.

COD_BIS is the record field that contains the code of another item that can
be sold in place of the current item if this is sold-out.

I'd like to add a constraint that verifies that COD_BIS is NULL or is
contained in the field COD_ART of a different record.

How do I create this script ?

Thank you

Duilio Foschi