create trigger chek_group_lessons
    BEFORE INSERT or UPDATE on sheldule
    for each row
    BEGIN
        if (EXISTS(SELECT * from sheldule where group_id = :NEW.group_id
        and lesson_id = :NEW.lesson_id
        and day_date = :NEW.lesson_id))
    THEN
        RAISE_APPLICATION_ERROR(-20000, 'Group already had lesson in this moment');
        return null;
    ELSE
        return :NEW;
    end if;

    end;