[原创]MAX函数和GROUP BY 语句一起使用的一个误区

使用MAX 函数和 GROUP 的时候会有不可预料的数据被SELECT 出来。
下面举个简单的例子：
想知道每个SCOREID 的 数学成绩最高的分数。


表信息：
/*DDL Information For - test.lkscore*/
--------------------------------------

Table    Create Table                                                                
-------  -----------------------------------------------------------------------------
lkscore  CREATE TABLE `lkscore` (                                                    
           `scoreid` int(11) DEFAULT NULL,                                           
           `chinese` int(11) DEFAULT '0',                                            
           `math` int(11) DEFAULT '0',                                               
           KEY `fk_class` (`scoreid`),                                               
           CONSTRAINT `fk_class` FOREIGN KEY (`scoreid`) REFERENCES `lkclass` (`id`) 
         ) ENGINE=InnoDB DEFAULT CHARSET=gb2312                                      

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		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。