mysql 的union 的用法  如果执行排序 分页操作  必须要把每个sql用括号括起来

$sql = "(select id,title,addtime,sender_id,receiver from fp_message_send where sender_id =".$user_id." and sender_role='1' and status < 8   )  union all(select id,receiver_id,receiver,send_id,status  from fp_message_receive where receiver_id= ".$user_id." and receiver_role = '1' and status < 8)  order by id DESC  limit ".$offset.",". $size;