Mysql max（）和min（）之间的区别？

Question

I got a problem with a mySql query and max() function. If I do :

Select * from Data group by experiment having min(timestamp)

This query return what I want, and correct value. I got this :

+----------+---------+----------+---------------------+----------------+------------+
| id       | mote_id | label_id | timestamp           | value          | experiment |
+----------+---------+----------+---------------------+----------------+------------+
|  3768806 |      10 |       30 | 2014-04-22 14:37:07 |              0 |         13 |
| 10989209 |      12 |       22 | 2014-04-25 10:44:03 | 2.532958984375 |         15 |
| 11943537 |       6 |       19 | 2014-05-05 17:20:15 |           1228 |         16 |
| 12042549 |      16 |       26 | 2014-05-06 10:48:59 |          22.86 |         17 |
| 12176642 |      15 |       23 | 2014-05-07 15:19:35 |              0 |         18 |
| 12195344 |      10 |        6 | 2014-05-07 15:27:23 |           3460 |         19 |
| 12222470 |      15 |        8 | 2014-05-07 15:38:38 |              1 |         21 |
| 12343934 |      10 |       19 | 2014-05-12 10:35:42 |            742 |         23 |
+----------+---------+----------+---------------------+----------------+------------+

But, if i do :

Select * from Data group by experiment having max(timestamp)

This query return wrong values... like this :

+----------+---------+----------+---------------------+----------------+------------+
| id       | mote_id | label_id | timestamp           | value          | experiment |
+----------+---------+----------+---------------------+----------------+------------+
|  3768806 |      10 |       30 | 2014-04-22 14:37:07 |              0 |         13 |
| 10989209 |      12 |       22 | 2014-04-25 10:44:03 | 2.532958984375 |         15 |
| 11943537 |       6 |       19 | 2014-05-05 17:20:15 |           1228 |         16 |
| 12042549 |      16 |       26 | 2014-05-06 10:48:59 |          22.86 |         17 |
| 12176642 |      15 |       23 | 2014-05-07 15:19:35 |              0 |         18 |
| 12195344 |      10 |        6 | 2014-05-07 15:27:23 |           3460 |         19 |
| 12222470 |      15 |        8 | 2014-05-07 15:38:38 |              1 |         21 |
| 12343934 |      10 |       19 | 2014-05-12 10:35:42 |            742 |         23 |
+----------+---------+----------+---------------------+----------------+------------+

In the first query, if I replace min(timestamp) by timestamp=min(timestamp) , it works, but in the second, "timestamp=max(timestamp)" return nothing

Finally, Select experiment,max(timestamp) return correct values.

mysql> select *,max(timestamp) from Data group by experiment;

+----------+---------+----------+---------------------+----------------+------------+---------------------+
| id       | mote_id | label_id | timestamp           | value          | experiment | max(timestamp)      |
+----------+---------+----------+---------------------+----------------+------------+---------------------+
|  3768806 |      10 |       30 | 2014-04-22 14:37:07 |              0 |         13 | 2014-04-24 16:03:29 |
| 10989209 |      12 |       22 | 2014-04-25 10:44:03 | 2.532958984375 |         15 | 2014-05-05 10:34:35 |
| 11943537 |       6 |       19 | 2014-05-05 17:20:15 |           1228 |         16 | 2014-05-06 10:35:15 |
| 12042549 |      16 |       26 | 2014-05-06 10:48:59 |          22.86 |         17 | 2014-05-07 15:19:33 |
| 12176642 |      15 |       23 | 2014-05-07 15:19:35 |              0 |         18 | 2014-05-07 15:27:23 |
| 12195344 |      10 |        6 | 2014-05-07 15:27:23 |           3460 |         19 | 2014-05-07 15:38:01 |
| 12222470 |      15 |        8 | 2014-05-07 15:38:38 |              1 |         21 | 2014-05-07 16:30:38 |
| 12343934 |      10 |       19 | 2014-05-12 10:35:42 |            742 |         23 | 2014-05-14 09:25:44 |
+----------+---------+----------+---------------------+----------------+------------+---------------------+

I know I can make a subquery to solve my probleme, but the tables contains thousands rows, and this solution is too long...

Ps : I can't use Select*, max(timestamp) even if it works because the query is run by EJB in JEE.

Answer 1

You select not determined values grouped by field experiment . No one can give you a guarantee that non-agregated fields would correspond to MIN or MAX values of some aggregated field.

You HAVE TO use sub-query or self-join to get the right records.

See more here: http://dev.mysql.com/doc/refman/5.6/en/example-maximum-column-group-row.html

Answer 2

The HAVING clause expects a boolean expression. In other DBMS your code sample would trigger an error. In MySQL, you'll get the expression cast to boolean:

• Zero → false
• Non-zero → true

And since your expression is constant for the whole set, it won't filter out partial rows.

As about this:

HAVING timestamp = max(timestamp)

The HAVING clause evaluates after WHERE and GROUP BY . At that point, using individual row values of the timestamp column doesn't make any sense. As usual, MySQL allows that but you must take into account that:

In standard SQL, a query that includes a GROUP BY clause cannot refer to nonaggregated columns in the HAVING clause that are not named in the GROUP BY clause. A MySQL extension permits references to such columns to simplify calculations. This extension assumes that the nongrouped columns will have the same group-wise values. Otherwise, the result is indeterminate.

In other words, your results are arbitrary (not even random).