mysql 分组排序并取前n条数据

参考地址：https://blog.csdn.net/weixin_39358657/article/details/89644822

通用模板：

select a.*
from
(
select t1.*,(select count(*)+1 from 表 where 分组字段=t1.分组字段 and 排序字段<t1.排序字段) as group_id
from 表 t1
) a
where a.group_id<=3  # 假设取前3条

举个栗子：

数据准备：

CREATE TABLE Score(
    s_id VARCHAR(20) COMMENT \'学生编号\',
    c_id  VARCHAR(20) COMMENT \'课程编号\',
    s_score INT(3) COMMENT \'分数\',
    PRIMARY KEY(s_id,c_id)
)ENGINE=InnoDB DEFAULT CHARSET=utf8 COMMENT \'成绩表\';

-- 成绩表测试数据
insert into Score values(\'01\' , \'01\' , 80);
insert into Score values(\'01\' , \'02\' , 90);
insert into Score values(\'01\' , \'03\' , 99);
insert into Score values(\'02\' , \'01\' , 70);
insert into Score values(\'02\' , \'02\' , 60);
insert into Score values(\'02\' , \'03\' , 80);
insert into Score values(\'03\' , \'01\' , 80);
insert into Score values(\'03\' , \'02\' , 80);

需求：查询每门功课成绩最好的前两名

需求分析：相当于查询课程为‘01’的前两名，课程为‘02’的排名前两名......

套用模板：

select a.s_id,a.c_id,a.s_score,a.group_id
from
(
select t1.*,(select count(*)+1 from score where c_id=t1.c_id and s_score>t1.s_score) as group_id
from score t1
) a
where a.group_id<=2 ORDER BY  c_id asc,group_id asc;

结果：

补充：

此处也可以使用下面的查询语句（前提是课程不多）

SELECT *FROM
(SELECT a.s_id,a.c_id,a.s_score, @i:=@i+1 as 排名 FROM score a,(SELECT @i:=0)b WHERE a.c_id=\'01\' ORDER BY a.s_score DESC
) c
WHERE 排名 BETWEEN 1 AND 2
UNION
SELECT *FROM
(SELECT a.s_id,a.c_id,a.s_score, @j:=@j+1 as 排名 FROM score a,(SELECT @j:=0)b WHERE a.c_id=\'02\' ORDER BY a.s_score DESC
) c
WHERE 排名 BETWEEN 1 AND 2
UNION
SELECT *FROM
(SELECT a.s_id,a.c_id,a.s_score, @k:=@k+1 as 排名 FROM score a,(SELECT @k:=0)b WHERE a.c_id=\'03\' ORDER BY a.s_score DESC
) c
WHERE 排名 BETWEEN 1 AND 2

上面的查询语句使用了变量@变量名：。

select @i:=@i+1 相当于对每一行进行添加序号；SELECT @i:=0设置初始值为0。