Question

I am really stuck on this. I’m clearly not understanding the MIN/MAX concept.

I am trying to get the latest row from a grouping of work_type and work_id.

If I change from MIN to MAX, it changes the returned timestamp, but it never brings the status info from that record.

Example:

"SELECT 
    CONCAT(work_type, work_id) AS condition_id, 
    status,
    MIN(created_timestamp) as latest
  FROM conditions
  GROUP BY condition_id"

With MIN, I get:

    Array
    (
        [0] => Array
            (
                [condition_id] => cutouts00002
                [status] => bad
                [latest] => 2011-02-21 15:20:27
            )

        [1] => Array
            (
                [condition_id] => paintings00002
                [status] => damagez
                [latest] => 2011-02-21 14:43:35
            )

    )

With MAX I get:

Array
(
    [0] => Array
        (
            [condition_id] => cutouts00002
            [status] => bad
            [latest] => 2011-02-21 15:22:20
        )

    [1] => Array
        (
            [condition_id] => paintings00002
            [status] => damagez
            [latest] => 2011-02-21 14:43:41
        )

)

Bu the thing is that the status in the row with the latest timestamp, is “no damage”, but it never returns the row corresponding to the MAX(current_timestamp), it only ever returns the “damagez” row.

Any help is appreciated.

Thanks.

Answer 1

You have fallen prey of the MySQL lax rules that allow for non-aggregates to be included in a GROUP BY query. Sure, you are working with MIN or MAX, and only ONE at a time, but consider this query:

SELECT 
    CONCAT(work_type, work_id) AS condition_id, 
    status,
    MIN(created_timestamp) as earliest,
    MAX(created_timestamp) as latest
  FROM conditions
  GROUP BY condition_id

Now, think about which row the status column should come from. It’s absurd to put a correlation between the aggregate (those in the GROUP BY) and non-aggregate columns.

Instead, write your query like this

SELECT X.condition_id, C.status, X.earliest
FROM (
  SELECT 
    CONCAT(work_type, work_id) AS condition_id, 
    status,
    MIN(created_timestamp) as earliest
  FROM conditions
  GROUP BY condition_id
) X JOIN conditions C
  on CONCAT(c.work_type, c.work_id) = X.condition_id
  and c.created_timestamp = X.earliest

But if you had two records with the same created_timestamp, it gets even more tricky

SELECT X.condition_id, Max(C.status) status, X.earliest
FROM (
  SELECT 
    CONCAT(work_type, work_id) AS condition_id, 
    status,
    MIN(created_timestamp) as earliest
  FROM conditions
  GROUP BY condition_id
) X JOIN conditions C
  on CONCAT(c.work_type, c.work_id) = X.condition_id
  and c.created_timestamp = X.earliest
GROUP BY X.condition_id, X.earliest