[MySQL] MySQL database queries on how groups after the last record in each group?

Original Address: https://codedefault.com/s/how-can-i-retrieve-the-last-record-in-each-group-mysql

Problem Description

For example, in MySQLthe database, data tables messagesand data records, as follows:

Id   Name   Other_Columns
-------------------------
1    A       A_data_1
2    A       A_data_2
3    A       A_data_3
4    B       B_data_1
5    B       B_data_2
6    C       C_data_1

If you perform MySQLgrouping query, as follows:

select * from messages group by name 

All will be returned by namethe first data packet, as follows:

1    A       A_data_1
4    B       B_data_1
6    C       C_data_1

So, how to query by namethe group return data in each of the last MySQLsentence of it, the returned results are as follows:

3    A       A_data_3
5    B       B_data_2
6    C       C_data_1

Option One

In MySQL version 8.0 before you can use the following statement:

WITH ranked_messages AS (
  SELECT m.*, ROW_NUMBER() OVER (PARTITION BY name ORDER BY id DESC) AS rn FROM messages AS m ) SELECT * FROM ranked_messages WHERE rn = 1; 

In MySQL version 8.0, MySQLadds support for window function (Window Functions), we can use window functions to simplify SQLthe query, and does not require the use GROUP BYclause, as follows:

WITH ranked_messages AS (
  SELECT m.*, ROW_NUMBER() OVER (PARTITION BY name ORDER BY id DESC) AS rn FROM messages AS m ) SELECT * FROM ranked_messages WHERE rn = 1; 

Option II

Use MySQLof the IN(...)clause, as follows:

SELECT id, name, other_columns
FROM messages
WHERE id IN ( SELECT MAX(id) FROM messages GROUP BY name ); 

third solution

Use MySQLof GROUP_CONCATand SUBSTRING_INDEXto query, as follows:

SELECT 
  `Id`,
  `Name`,
  SUBSTRING_INDEX(
    GROUP_CONCAT(
      `Other_Columns` ORDER BY `Id` DESC SEPARATOR '||' ), '||', 1 ) Other_Columns FROM messages GROUP BY `Name`

Practice Code:

 1 SELECT * from(
 2 SELECT
 3 ( @i := CASE WHEN @pre_parent_code = sem_shop_wangwang THEN @i + 1 ELSE 1 END ) rownum,t_sem_logs.*,
 4 ( @pre_parent_code := sem_shop_wangwang ) 
 5 FROM
 6             t_sem_logs,
 7             ( SELECT @i := 0, @pre_parent_code := '' ) AS b
 8             where sem_edit_time <= '2018-07-13'
 9 GROUP BY 
10             sem_shop_wangwang,id  
11         ORDER BY sem_shop_wangwang desc,DATE_FORMAT(sem_edit_time,'%Y-%m-%d %H:%i:%s') DESC,sem_agree_state ASC
12 ) a where rownum = 1   GROUP BY sem_agree_state order by DATE_FORMAT(sem_edit_time,'%Y-%m-%d %H:%m:%s') DESC;
13