Incorrect usage of UNION and ORDER BY

UNION 和 ORDER BY 一起使用

1.需求分組排序完    再合併

(   select * from (
                select a from a1  order by a2
     ) as table1
) 
union 
(   select * from ( 
                 select b from b1 order by b2 
    ) as table2 
) 

2.整體合併     再排序

select  *  from

（...   union    ....）   as  a    【派生表別忘了加別名，否則 Every derived table must have its own alias】

order  by..

order by 別忘了在 select語句中查詢出

 

union和union all的差別是前者會忽略重複數據