如何在同一张表的不同组的行之间进行计算

如何在同一张表的不同组的行之间进行计算

您的查询无效，因为您在表上执行

CROSS JOIN

（是因为

,

），因此每一行都与另一个表中的每一行都匹配，而不是

INNER JOIN

一个月匹配。

修改查询：

select a.month, a.value-b.valuefrom  (select month, value from table where year = 2012) a  JOIN  (select month, value from table where year = 2011) b  ON a.month = b.month

更快的查询：

select a.month, a.value-b.valuefrom  yourTable a  join yourTable b    on a.month = b.month  where a.year = 2012 and b.year = 2011

每年每月多行：

select a.month, a.value-b.valuefrom  (select month, sum(value) as value   from yourTable where year = 2012   group by month) a  join  (select month, sum(value) as value   from yourTable where year = 2011   group by month) b    on a.month = b.month

SQLFiddle。