在MySQL使用中,经常需要查询每个 分组的前几条记录(查询分组后每一个组的前几项),下面写了个简单的例子说明下SQL的写法。简单的表设计如下,要求每个班总分排名最前的前两条数据。
测试表语句如下:
1 | create table test(id int unsigned not null auto_increment primary key , name varchar (10),class varchar (20),score varchar (20)); |
2 | insert into test( name , class, score) values ( 'gonn' , '6(1)' , '299' ); |
3 | insert into test( name , class, score) values ( 'yyun' , '6(1)' , '259' ); |
4 | insert into test( name , class, score) values ( 'lin' , '6(1)' , '289' ); |
5 | insert into test( name , class, score) values ( 'mei' , '6(1)' , '277' ); |
6 | insert into test( name , class, score) values ( 'xj' , '6(2)' , '287' ); |
7 | insert into test( name , class, score) values ( 'zhl' , '6(2)' , '277' ); |
8 | insert into test( name , class, score) values ( 'lwjs' , '6(2)' , '257' ); |
9 | insert into test( name , class, score) values ( 'lulu' , '6(2)' , '265' ); |
运行以上SQL,得到的表结构如下:
01 | mysql> SELECT * FROM test; |
02 | + ----+------+-------+-------+ |
03 | | id | name | class | score | |
04 | + ----+------+-------+-------+ |
05 | | 1 | gonn | 6(1) | 299 | |
06 | | 2 | yyun | 6(1) | 259 | |
07 | | 3 | lin | 6(1) | 289 | |
08 | | 4 | mei | 6(1) | 277 | |
09 | | 5 | xj | 6(2) | 287 | |
10 | | 6 | zhl | 6(2) | 277 | |
11 | | 7 | lwjs | 6(2) | 257 | |
12 | | 8 | lulu | 6(2) | 265 | |
13 | + ----+------+-------+-------+ |
方法一
01 | mysql> SELECT a.id,a. name ,a.class,a.score |
02 | FROM test a LEFT JOIN test b on a.class = b.class and a.score < b.score |
03 | GROUP BY a.id,a. name ,a.class,a.score |
05 | ORDER BY a.class,a.score DESC ; |
06 | + ----+------+-------+-------+ |
07 | | id | name | class | score | |
08 | + ----+------+-------+-------+ |
09 | | 1 | gonn | 6(1) | 299 | |
10 | | 3 | lin | 6(1) | 289 | |
11 | | 5 | xj | 6(2) | 287 | |
12 | | 6 | zhl | 6(2) | 277 | |
13 | + ----+------+-------+-------+ |
方法二
01 | mysql> SELECT * FROM test a |
02 | WHERE 2 >( SELECT count (*) FROM test WHERE class = a.class and score>a.score) |
03 | ORDER BY a.class,a.score DESC ; |
04 | + ----+------+-------+-------+ |
05 | | id | name | class | score | |
06 | + ----+------+-------+-------+ |
07 | | 1 | gonn | 6(1) | 299 | |
08 | | 3 | lin | 6(1) | 289 | |
09 | | 5 | xj | 6(2) | 287 | |
10 | | 6 | zhl | 6(2) | 277 | |
11 | + ----+------+-------+-------+ |
这里列出了多种SQL语句的实现方法,有些是MySQL特有的(Limit, 其它数据库可根据实际更改,比如oracle的rownum,MS SQL SERVER 的 top,..),有时是SQL标准支持的。但效率上和应用的场合或许不同。具体应用时可根据实际表中的记录情况,索引情况进行选择。
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