UNION 和 UNION ALL的区别，一个例子就看明白

【UNION ALL】

select a,b,sum(sm) AS s1, SUM(qm) AS s2 from 
(
 select 'a' AS a, 'b' AS b, 2 AS sm, 200 AS qm 
UNION ALL
 select 'a' AS a, 'b' AS b, 2 AS sm, 200 AS qm
)r
group by a,b

 ab4400

【UNION】

select a,b,sum(sm) AS s1, SUM(qm) AS s2 from 
(
 select 'a' AS a, 'b' AS b, 2 AS sm, 200 AS qm 
UNION 
 select 'a' AS a, 'b' AS b, 2 AS sm, 200 AS qm
)r
group by a,b

 ab2200

 

总结：UNION ALL全部列出，UNION会去掉重复的。