I know nothing about Firebird 2.0, but the below statement does refer to
'a1' (ai.id) which is a non-existing table.

HTH,
Set

> /**********************************/
>
> ALTER TABLE S1 ADD BASE COMPUTED BY (
> cast(
> coalesce(
> (select sum(cast(s2.cantidad*s2.Precio as numeric(14,4))
> *cast((1.0-s2.dto/100)*(1.0-s1.dto/100) as numeric(6,5)))
> from s2
> where a1.id = s2.id)
> ,0)
> as numeric(14,2)));
>
> /**********************************/