Question

Data: (log_time is a DATETIME type)

log_id  | action      | log_time            | user
--------------------------------------------------
1         Processed     2011-02-28 16:38:48   1
2         Processed     2011-03-02 16:56:43   5
3         Processed     2011-03-02 17:00:17   5
4         Processed     2011-03-03 08:59:33   5

Query:

SELECT log_time, user 
FROM logs
WHERE action = "Processed"
GROUP BY action 
HAVING MAX(log_time)

Result:

log_time            | user
--------------------------
2011-02-28 16:38:48   1

Clearly, this is not having the max log_time at all. If I change the query to…

SELECT MAX(log_time), user 
FROM logs 
WHERE action = "Processed" 

Then I get, naturally:

log_time            | user
--------------------------
2011-03-03 08:59:33   1

Now, the data I obviously want is the data in row 4: March 3, but user 5. I understand that I can get this by doing a simple SELECT ... ORDER BY log_time DESC LIMIT 1. But my question is, what am I doing with these MAX() queries that isn’t correct? It would seem to me that if I ran a query with a HAVING MAX() that it would give me the row that, well, had the max. What am I not understanding about how MAX() works?

Edit: To elaborate my question, basically, when I see a query…

SELECT * FROM logs WHERE action = "Processed"
GROUP BY action HAVING MAX(log_time)

… my assumption, based on how the code appears is that it will retrieve the row with the largest log_time where action is Processed. This appears to be a faulty assumption. What, then, does HAVING MAX() even mean?

Answer 1

Based on the input of others, particularly Damien_The_Unbeliever, I realized that my problem was that HAVING MAX() doesn’t actually do anything. It will just pipe out the date, which doesn’t work as it’s not being compared to anything.

When I say HAVING MAX(log_time), that translates to HAVING 2011-03-03 08:59:33, which doesn’t tell the SQL what it’s supposed to have, it’s just a statement, like IF (5). I think. HAVING continues to be somewhat of a mystery for me, but I think this is the reason why this particular issue was causing me grief.