转载鑫语大数据的 微信存不了收藏夹 复制过来的 侵权立删
CREATE TABLE `test1` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`name` varchar(20) DEFAULT NULL,
`course` varchar(20) DEFAULT NULL,
`score` int(11) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=10 DEFAULT CHARSET=utf8;
insert into test1(name,course,score)
values
('张三','语文',80),
('李四','语文',90),
('王五','语文',93),
('张三','数学',77),
('李四','数学',68),
('王五','数学',99),
('张三','英语',90),
('李四','英语',50),
('王五','英语',89);
select * from test1;
Top 1
查询每门课程分数最高的学生以及成绩
1、使用自连接【推荐】
select a.name,a.course,a.score from test1 a
join (select course,max(score) score from test1 group by course) b
on a.course=b.course and a.score=b.score;
2、使用相关子查询
select `name`,course,score from test1 a
where score=(select max(score) from test1 where a.course=test1.course);
select `name`,course,score from test1 a
where not exists
(select 1 from test1 where a.course=test1.course and a.score < test1.score);
Top N
N>=1
查询每门课程前两名的学生以及成绩
1、使用union all
如果结果集比较小，可以用程序查询单个分组结果后拼凑，也可以使用union all
(select name,course,score from test1 where course='语文' order by score desc limit 2) union all
(select name,course,score from test1 where course='数学' order by score desc limit 2) union all
(select name,course,score from test1 where course='英语' order by score desc limit 2);
2、自身左连接
select a.name,a.course,a.score
from test1 a left join test1 b on a.course=b.course and a.score<b.score
group by a.name,a.course,a.score
having count(b.id)<2
order by a.course,a.score desc;
3、相关子查询
select * from test1 a
where 2>(select count(*) from test1 where course=a.course and score>a.score)
order by a.course,a.score desc;
4、使用用户变量
set @num := 0, @course := '';
select name, course, score
from (
select name, course, score,
@num := if(@course = course, @num + 1, 1) as row_number,
@course := course as dummy
from test1 order by course, score desc) as x
where x.row_number <= 2;
备注：
如果是oracel
SELECT * FROM (SELECT ROW_NUMBER() OVER(PARTITION BY x ORDER BY y DESC)
rn,test1.* FROM test1)
WHERE rn = 1 ;