CREATE SEQUENCE "employee_number_seq"
    START WITH 1
    INCREMENT BY 1
    NO MINVALUE
    NO MAXVALUE
    CACHE 1;

DO $$
DECLARE
  v_next_value INTEGER;
BEGIN
  SELECT COALESCE(MAX(SUBSTRING(employee_no FROM 4)::INTEGER), 0) + 1
    INTO v_next_value
    FROM employees
   WHERE employee_no ~ '^HYD[0-9]{6}$';

  EXECUTE format('ALTER SEQUENCE "employee_number_seq" RESTART WITH %s', v_next_value);
END;
$$;

CREATE OR REPLACE FUNCTION generate_employee_number()
RETURNS TEXT
LANGUAGE plpgsql
AS $$
DECLARE
  v_seq INTEGER;
BEGIN
  v_seq := nextval('"employee_number_seq"');
  IF v_seq > 999999 THEN
    RAISE EXCEPTION '员工编号序列已达上限 999999';
  END IF;
  RETURN 'HYD' || LPAD(v_seq::TEXT, 6, '0');
END;
$$;

ALTER TABLE "employees"
  ALTER COLUMN "employee_no" SET DEFAULT generate_employee_number();

CREATE OR REPLACE FUNCTION prevent_employee_no_update()
RETURNS trigger
LANGUAGE plpgsql
AS $$
BEGIN
  IF OLD.employee_no IS DISTINCT FROM NEW.employee_no THEN
    RAISE EXCEPTION '禁止修改员工编号：employee_no 是不可变字段';
  END IF;
  RETURN NEW;
END;
$$;

CREATE TRIGGER prevent_employee_no_update_trigger
BEFORE UPDATE ON "employees"
FOR EACH ROW
EXECUTE FUNCTION prevent_employee_no_update();