CREATE OR REPLACE FUNCTION process_chek_group_lessons()
RETURNS TRIGGER AS
$$

BEGIN
    IF(EXISTS(SELECT * FROM sheldule WHERE GROUP_ID = NEW.GROUP_ID
        AND lesson_id = NEW.lesson_id
        AND day_date = NEW.day_date))
    THEN
        RAISE EXCEPTION 'Group cannod had two lessons in one time';
        RETURN NULL;
    ELSE
        RETURN NEW;
    END IF;
END;
$$
LANGUAGE plpgsql;

CREATE TRIGGER chek_group_lessons
BEFORE INSERT OR UPDATE ON sheldule
    FOR EACH ROW EXECUTE FUNCTION process_chek_group_lessons()