Question

I have a table in which I need to keep the total number of rows within 50,000. This table includes an Id field (id is auto incremental).

How can I check if max(id) - min(id) > 50,000, then delete the earlier rows?

DELETE FROM news WHERE if (max(id) - min(id) > 50000) 

This query will delete all the rows if max(id) - min(id) > 50000, what is the correct way? Ideally I need a one line command, excuse in SSH method. thanks.

Answer 1

DELETE d 
FROM news AS d
  JOIN
    ( SELECT MAX(id)-50000 AS lim 
      FROM news
    ) AS m
    ON d.id < m.lim ;

The above will not leave exactly 50K rows of course, as there may be gaps in the id sequence. But I guess this is expected and not a problem. If you really want to leave exactly 50K rows, any statement will probably be less efficient. You can try this one:

DELETE d 
FROM news AS d
  JOIN
    ( SELECT id AS lim 
      FROM news
      ORDER BY id DESC
      LIMIT 1 OFFSET 50000
    ) AS m
    ON d.id <= m.lim ;