mysql分组取topN的三种方式

目录：

一、需求

二、建表语句插入数据

三、表内容展示

四、结果展示

五、三种mySQL查询的方式

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一、需求

有表 如下，要求取出各班前两名（允许并列）

二、建表语句插入数据

CREATE TABLE topn_yh(
     id int not null primary key,
    SName VARCHAR(20),
	ClsNo VARCHAR(20),
	Score VARCHAR(20)
)ENGINE=InnoDB DEFAULT CHARSET=utf8;

INSERT INTO topn_yh(id , SName,ClsNo,Score) values (1,'AAAA','C1','67');
INSERT INTO topn_yh(id , SName,ClsNo,Score) values (2,'BBBB','C1','55');
INSERT INTO topn_yh(id , SName,ClsNo,Score) values (3,'CCCC','C1','67');
INSERT INTO topn_yh(id , SName,ClsNo,Score) values (4,'DDDD','C1','65');
INSERT INTO topn_yh(id , SName,ClsNo,Score) values (5,'EEEE','C1','95');
INSERT INTO topn_yh(id , SName,ClsNo,Score) values (6,'FFFF','C2','57');
INSERT INTO topn_yh(id , SName,ClsNo,Score) values (7,'GGGG','C2','87');
INSERT INTO topn_yh(id , SName,ClsNo,Score) values (8,'HHHH','C2','74');
INSERT INTO topn_yh(id , SName,ClsNo,Score) values (9,'IIII','C2','52');
INSERT INTO topn_yh(id , SName,ClsNo,Score) values (10,'JJJJ','C2','81');
INSERT INTO topn_yh(id , SName,ClsNo,Score) values (11,'KKKK','C2','67');
INSERT INTO topn_yh(id , SName,ClsNo,Score) values (12,'LLLL','C2','66');
INSERT INTO topn_yh(id , SName,ClsNo,Score) values (13,'MMMM','C2','63');
INSERT INTO topn_yh(id , SName,ClsNo,Score) values (14,'NNNN','C3','99');
INSERT INTO topn_yh(id , SName,ClsNo,Score) values (15,'OOOO','C3','50');
INSERT INTO topn_yh(id , SName,ClsNo,Score) values (16,'PPPP','C3','59');
INSERT INTO topn_yh(id , SName,ClsNo,Score) values (17,'QQQQ','C3','66');
INSERT INTO topn_yh(id , SName,ClsNo,Score) values (18,'RRRR','C3','76');
INSERT INTO topn_yh(id , SName,ClsNo,Score) values (19,'SSSS','C3','50');
INSERT INTO topn_yh(id , SName,ClsNo,Score) values (20,'TTTT','C3','50');
INSERT INTO topn_yh(id , SName,ClsNo,Score) values (21,'UUUU','C3','64');
INSERT INTO topn_yh(id , SName,ClsNo,Score) values (22,'VVVV','C3','74');

三、表内容展示

id	SName	ClsNo	Score
1	AAAA	C1	67
2	BBBB	C1	55
3	CCCC	C1	67
4	DDDD	C1	65
5	EEEE	C1	95
6	FFFF	C2	57
7	GGGG	C2	87
8	HHHH	C2	74
9	IIII	C2	52
10	JJJJ	C2	81
11	KKKK	C2	67
12	LLLL	C2	66
13	MMMM	C2	63
14	NNNN	C3	99
15	OOOO	C3	50
16	PPPP	C3	59
17	QQQQ	C3	66
18	RRRR	C3	76
19	SSSS	C3	50
20	TTTT	C3	50
21	UUUU	C3	64
22	VVVV	C3	74

四、结果展示

id	SName	ClsNo	Score
5	EEEE	C1	95
1	AAAA	C1	67
3	CCCC	C1	67
7	GGGG	C2	87
10	JJJJ	C2	81
14	NNNN	C3	99
18	RRRR	C3	76

五、三种mySQL查询的方式

方法一：

select a.id,a.SName,a.ClsNo,a.Score
from topn_yh a left join topn_yh b on a.ClsNo=b.ClsNo and a.Score<b.Score
group by a.id,a.SName,a.ClsNo,a.Score
having count(b.id)<2
order by a.ClsNo,a.Score desc

方法二：

select *
from topn_yh a
where 2>(select count(*) from topn_yh where ClsNo=a.ClsNo and Score>a.Score)
order by a.ClsNo,a.Score desc

方法三：

select *
from topn_yh a
where id in (select id from topn_yh where ClsNo=a.ClsNo order by Score desc limit 2)
order by a.ClsNo,a.Score desc