MySQL uses GROUP BY grouping to implement the method of fetching the first N records_MySQL

The example in this article describes how mysql uses GROUP BY grouping to achieve the first N records. Share it with everyone for your reference, the details are as follows:

GROUP BY grouping in MySQL is implemented by taking the first N records

mysql grouping, fetching records

How to get the first two digits of each group after GROUP BY. Now I will tell you how to get the first N records of GROUP BY grouping in mysql.

This is the test table (I don’t know what I thought, I typed aa in the table name at that time, sweat~~~~):

Result:

Method 1:

The code is as follows:

SELECT a.id,a.SName,a.ClsNo,a.Score FROM aa a LEFT JOIN aa 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;

Teardown analysis:

1、 LEFT JOIN aa b ON a.ClsNo=b.ClsNo AND a.Score

In the same class (four students in each class), records with higher scores than the current student will mean that the student with the lowest score will have three records

2. group by a.id,a.SName,a.ClsNo,a.Score having count (b.id)<2
a.id, a.SName, a.ClsNo, a.Score can represent a student (grouped by students), if count(b.id)<2 (no more than 2 people can exceed your score), then Only first and second are left.

Method 2:

The code is as follows:

SELECT * FROM aa a WHERE 2>(SELECT COUNT(*) FROM aa WHERE ClsNo=a.ClsNo and Score>a.Score) ORDER BY a.ClsNo, a.Score DESC;

I think this is more interesting. Take each record and determine whether there are less than 2 students in the same class who have higher grades than the current one.

Method three:

The code is as follows:

SELECT * FROM aa WHERE id IN (SELECT id FROM aa WHERE ClsNo=a.ClsNo ORDER BY Score DESC LIMIT 2) ORDER BY a.ClsNo,a.Score DESC ;

This method failed to pass the test, ERROR 1235 (42000): This version of MySQL doesn't yet support 'LIMIT & IN/ALL/ANY/SOME subquery', limit cannot be used in these subqueries.

Readers who are interested in more MySQL-related content can check out the special topics on this site: "A Complete Collection of MySQL Log Operation Skills", "A Summary of MySQL Transaction Operation Skills", "A Complete Collection of MySQL Stored Procedure Skills", and "A Summary of MySQL Database Lock Related Skills" 》and《Summary of commonly used functions in MySQL》

I hope this article will be helpful to everyone in MySQL database planning.