$sql = "
SELECT 'ISSUE' type, order_date dt, gascode, cylinder_count_qty qty
FROM order_trans
WHERE custcode=? AND gascode=? AND DATE_FORMAT(order_date,'%Y-%m')=?

UNION ALL

SELECT 'RETURN', entry_date, gascode, cylinder_count_qty
FROM return_trans
WHERE custcode=? AND gascode=? AND DATE_FORMAT(entry_date,'%Y-%m')=?
ORDER BY dt
";