create or replace trigger Banks_delete
before delete on banks
for each row
declare
    c number;
begin
SELECT COUNT(*) INTO c FROM accounts WHERE bank_id=:old.bank_id and dayto>sysdate;
    IF(c<2) THEN
       update accounts Set bank_id=null where bank_id IN (SELECT bank_id FROM accounts WHERE bank_id=:OLD.bank_id);
    ELSE
        raise_application_error(-20000,'Банк '||:old.name||' имеет более 1 действующего контракта.');
    END IF;
end;

delete from banks where bank_id=1 or bank_id=5