Question

I have a table that contains

id  username    password    last_touch

It is possible to have duplicate entries for the same id. The last_touch is the timestamp of the last insert command. How can I delete all entries with the same id, apart from the last one that is left so I always have the user details which are most up to date?

Somewhere along the lines of:

DELETE FROM user_data 
WHERE id=1 
LIMIT (count(SELECT 1 
             FROM user_data 
             WHERE id=1) - 1)

(Obviously the syntax is not correct in the above example, so MySQL complains.)

Answer 1

Use a nested query to select the latest timestamp for the given user id, and delete all rows for that id with timestamps different than that one:

DELETE FROM user_data
WHERE ID = 1 AND last_touch != 
    (SELECT latest FROM
        (SELECT MAX(last_touch) AS latest FROM user_data WHERE ID = 1) AS temp
    )

Update: Fixed direct reference to table being modified in inner SELECT by wrapping another SELECT around it, as per Frank’s comments.