CREATE OR REPLACE FUNCTION check_salary_positive()
RETURNS TRIGGER AS $$
BEGIN
    IF NEW.salary < 0 THEN
        RAISE EXCEPTION 'Зарплата не может быть отрицательной!';
    END IF;
    RETURN NEW;
END;
$$ LANGUAGE plpgsql;

CREATE TRIGGER trg_check_salary
BEFORE INSERT OR UPDATE ON employees
FOR EACH ROW
EXECUTE FUNCTION check_salary_positive();