union和union all區別

UNION會把兩個查詢語句的結果集合並起來。
union會過濾掉兩個結果集中重複的行，而union all不會過濾掉重複行。

union order

在union兩個帶order的select的時候，會報錯。

ERROR 1221 (HY000): Incorrect usage of UNION and ORDER BY

解決方法就是使用括號，把兩個SELECT括起來。
例如：

(SELECT A,B,C FROM `s` ORDER BY A DESC) 
UNION 
(SELECT A,B,C FROM `r` ORDER
 
 BY B DESC);

或用SELECT AS TABLE

SELECT * FROM 
(SELECT A,B,C FROM `s` ORDER BY A DESC)
 AS t1 
 UNION 
 SELECT A,B,C FROM `r` ORDER BY B DESC;