Are you trying to emulate a boolean data type in Firebird? If so, a
domain will be your ticket (until such time as Firebird implements
booleans, that is):

CREATE DOMAIN D_BOOLEAN AS SMALLINT
CHECK (VALUE IS NULL OR VALUE IN (-1, 0));

This places the desired restriction on any columns defined using the
domain. Make sure, of course, that you don't actually name the domain
BOOLEAN, since this is a Firebird reserved word.

-Marc Benedict