MAX函数和GROUP BY 语句一起使用的一个误区

摘自yueliangdao0608.cublog.cn
http://blog.chinaunix.net/u/29134/
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 <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。