create trigger blokada_ad_pres
on employees
for insert, update
as
begin
declare @job varchar (max)

select @job = job_id from inserted

if exists ((select job_id from employees where job_id like 'AD_PRES') and (@job_id = 'AD_PRES'))
    begin
    print 'Istnieje już pracownik o job_id = ADPRES'
    rollback transaction
    end

else
    begin
    print 'dodano nowego pracownika'
    end

end