Deleting Duplicates in MySQL

Question

I have a table like this

userid  visitorid   time
1       10          2009-12-23
1       18          2009-12-06
1       18          2009-12-14
1       18             


        

Answer 1

Assuming your table is called Visitors:

DELETE v1.* FROM Visitors v1
LEFT JOIN (
    SELECT userid, visitorid, MAX(time) AS time
    FROM Visitors v2
    GROUP BY userid, visitorid
) v3 ON v1.userid=v3.userid AND v1.visitorid=v3.visitorid AND v1.time = v3.time
WHERE v3.userid IS NULL;