MySQL query, MAX() + GROUP BY

Question

Daft SQL question. I have a table like so (\'pid\' is auto-increment primary col)

CREATE TABLE theTable (
    `pid` INT UNSIGNED PRIMARY KEY AUTO_INCREMENT,
          


        

Answer 1

I created an index on rid and timestamp.

SELECT test.pid, test.cost, test.timestamp, test.rid
FROM theTable AS test
LEFT JOIN theTable maxt 
ON maxt.rid = test.rid
AND maxt.timestamp > test.timestamp
WHERE maxt.rid IS NULL 

Showing rows 0 - 2 (3 total, Query took 0.0104 sec)

This method will select all the desired values from theTable (test), left joining itself (maxt) on all timestamps higher than the one on test with the same rid. When the timestamp is already the highest one on test there are no matches on maxt - which is what we are looking for - values on maxt become NULL. Now we use the WHERE clause maxt.rid IS NULL or any other column on maxt.