Question

Sorry for the ambiguous title; but I don’t now how to describe it different.

I have the following table:

imei | date       | time  | some more fields
345  | 2012-06-28 | 07:18 | .....
345  | 2012-06-28 | 07:20 | .....
345  | 2012-06-28 | 07:21 | .....
987  | 2012-06-28 | 07:19 | .....
etc etc

I want to get the latest row of ervery distinct imei, so:

345  | 2012-06-28 | 07:21
987  | 2012-06-28 | 07:19

• Using SELECT * FROM t GROUP BY imei results in using the first line instead of the last one.
• Ordering by time obviously orders the result relation instead of the sub.
• Using having is only for stating a condition….

How can I write a query to do this?

Answer 1

As stated in the manual:

The server is free to choose any value from each group, so unless they are the same, the values chosen are indeterminate. Furthermore, the selection of values from each group cannot be influenced by adding an ORDER BY clause. Sorting of the result set occurs after values have been chosen, and ORDER BY does not affect which values the server chooses.

To obtain the groupwise maximum (as you want), you need to join the result of a grouped subquery with your table:

SELECT * FROM t JOIN (
  SELECT imei, MAX(ADDTIME(date, time)) AS dt FROM t GROUP BY imei
) AS u ON t.imei = u.imei AND t.date = DATE(u.dt) AND t.time = TIME(u.dt)