MySQL ORDER BY

　　注意点：

　　　　1，与 Union 同用时需注意：

           select id from test1 order by id

           union all

           select id from test2 order by id

　　　　以上用法不合法；正确用法如下：

           select * from (select id from test1 order by id) t1

           union all

           select * from (select id from test2 order by id) t2