MySQL 5.7 takes the first item of the group

Here is an example , Need to take every class 、 The discipline selected the most times , such as 1 There are three students in class who choose math , One who chooses Chinese , So what you expect to return is Mathematics , Others in the same way .

id	name	class	course
1	Zhang San	1	mathematics
2	Li Si	1	mathematics
3	Wang Wu	1	mathematics
4	Zhao Liu	1	Chinese language and literature
5	Black seven	2	Chinese language and literature
6	Pig eight	2	Chinese language and literature
7	Qin Jiu	2	mathematics
8	Wang Shi	3	mathematics
9	Mao Xi	3	mathematics
10	Xiao twelve	3	mathematics
11	A thirteen	3	Chinese language and literature

The first method ： limit + group by

example

SELECT t.* 
FROM
	( 
		SELECT ta.class,ta.course,count(*) as counts
		FROM courses as ta 
		group by ta.class,ta.course
		order by counts desc //  Sort here 
		LIMIT 1000 // It's necessary here , And it seems that the maximum value is 1 ten thousand 
	) t 
GROUP BY t.class // The outer layer uses grouping 

The results are as follows

class	course	counts
1	mathematics	3
2	Chinese language and literature	2
3	mathematics	3

I found several blog posts in the station , It's basically this routine , Maybe this method will perform better , And it seems that you can directly get the whole row of data with the maximum value , It's very convenient .
For example, this blog post also analyzes optimization ：https://blog.csdn.net/u013066244/article/details/116461584
however ： Because the company server is enabled ONLY_FULL_GROUP_BY, So I can't use it .

The second method ：group_concat+substring_index

select tb.class,SUBSTRING_INDEX(GROUP_CONCAT(tb.course order by counts desc),',',1) as course
from 
	(
		SELECT ta.class,ta.course,count(*) as counts
		FROM courses as ta 
		group by ta.class,ta.course
	) as tb 
group by tb.class;

The results are as follows

class	course
1	mathematics
2	Chinese language and literature
3	mathematics

About this method , It has been written by bloggers , Personally, I think it's very detailed , Connect here :https://blog.csdn.net/MyfishCake/article/details/120175776