推荐：MAX函数和GROUP BY 语句一起使用的一个误区

 

select * from lkscore;

query result(12 records)

scoreid	chinese	math
1	90	80
2	100	99
3	29	98
4	87	79
5	89	99
1	49	98
3	98	56
2	76	88
2	80	90
3	90	70
1	90	90
1	67	90

错误的SELECT

select scoreid,chinese,max(math) max_math from lkscore group by scoreid;

query result(5 records)

scoreid	chinese	max_math
1	90	98
2	100	99
3	29	98
4	87	79
5	89	99

上面的90明显不对。

方法一：

select scoreid,chinese,math max_math from ( select * from lkscore order by math desc ) T group by scoreid;

query result(5 records)

scoreid	chinese	max_math
1	49	98
2	100	99
3	29	98
4	87	79
5	89	99

方法二：

select * from lkscore a where a.math = (select max(math) from lkscore where scoreid = a.scoreid) order by scoreid asc;

query result(5 records)

scoreid	chinese	max_math
1	49	98
2	100	99
3	29	98
4	87	79
5	89	99

这个也是用MAX函数，而且还用到了相关子查询。
我们来看一下这两个的效率如何：

explain select scoreid,chinese,math max_math from (select * from lkscore order by math desc) T group by scoreid;

query result(2 records)

id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	PRIMARY	<derived2>	ALL	(NULL)	(NULL)	(NULL)	(NULL)	12	Using temporary; Using filesort
2	DERIVED	lkscore	ALL	(NULL)	(NULL)	(NULL)	(NULL)	12	Using filesort

很明显，有两个FULL TABLE SCAN。

explain select scoreid,chinese,math max_math from lkscore a where a.math = (select max(math) from lkscore where scoreid = a.scoreid) order by scoreid asc;

query result(2 records)

id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	PRIMARY	a	index	(NULL)	fk_class	5	(NULL)	12	Using where
2	DEPENDENT SUBQUERY	lkscore	ref	fk_class	fk_class	5	a.scoreid	1	Using where

第二个就用了KEY,子查询里只扫描了一跳记录。

很明显。在这种情况下第二个比第一个效率高点。

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推荐：MAX函数和GROUP BY 语句一起使用的一个误区http://yueliangdao0608.blog.51cto.com/397025/81278