mysql - Delete all rows from a MYSQL table except TOP/BOTTOM 50 rows

Question

http://sqlfiddle.com/#!2/d21f3/1

I have a table with some entries here, I want to keep only 50 messages in this table sorted by message_id, and DELETE the rest of entries.

Please help me with the query.

Thanks in advance.

Answer 1

试试我用过ORDER BY message_id DESC的这个，随心所欲地改变它，它将删除除已选择的 50 个条目之外的所有条目，我为查询设置了别名，因为您不能使用同一个表来选择删除操作

DELETE FROM `chat_history` WHERE id NOT IN ( SELECT t.id FROM 
(SELECT id FROM chat_history ORDER BY message_id DESC LIMIT 50 ) t)

Answer 2

例如。

DELETE a 
  FROM chat_history a 
  LEFT 
  JOIN 
     ( SELECT x.message_id 
         FROM chat_history x 
         JOIN chat_history y 
           ON y.message_id >= x.message_id 
        GROUP 
           BY x.message_id 
       HAVING COUNT(*) <= 50
     ) b 
    ON b.message_id = a.message_id 
 WHERE b.message_id IS NULL;

http://sqlfiddle.com/#!2/361b4/1