Question

Given following Table

select rv, mv, datetime, rawv from mytable where datetime > '2012-01-30';
+------+------+---------------------+------+
| rv   | mv   | datetime            | rawv |
+------+------+---------------------+------+
|    5 |   81 | 2012-01-31 07:00:00 |    1 |
|    5 |   81 | 2012-01-31 07:01:00 |    2 |
|    5 |   82 | 2012-01-31 07:00:00 |    3 |
|    5 |   82 | 2012-01-31 07:01:00 |    4 |
|    5 |   83 | 2012-01-31 08:00:00 |    5 |
|    5 |   83 | 2012-01-31 08:01:00 |    6 |
+------+------+---------------------+------+

I’m looking for a SQL-statement which provides the records with the maximal datetime value for each rv, mv group. The result should look like:

+------+------+---------------------+------+
| rv   | mv   | max(datetime)       | rawv |
+------+------+---------------------+------+
|    5 |   81 | 2012-01-31 07:01:00 |    2 |
|    5 |   82 | 2012-01-31 07:01:00 |    4 |
|    5 |   83 | 2012-01-31 08:01:00 |    6 |
+------+------+---------------------+------+

Do you know such a statement?

What I’ve tried so far is following, but as you can see I’m getting wrong results:

select rv, mv, max(datetime), rawv from mytable
where mv in (81,82,83)
group by rv, mv;
+------+------+---------------------+------+
| rv   | mv   | max(datetime)       | rawv |
+------+------+---------------------+------+
|    5 |   81 | 2012-01-31 07:01:00 |    0 |
|    5 |   82 | 2012-01-31 07:01:00 |    0 |
|    5 |   83 | 2012-01-31 08:01:00 |    5 |
+------+------+---------------------+------+
3 rows in set (0.72 sec)

and

select rv, mv, datetime, rawv from mytable
where mp_nb in (81,82,83)
and datetime=(select max(datetime) from met_value);

+------+------+---------------------+------+
| rv   | mv   | datetime            | rawv |
+------+------+---------------------+------+
|    5 |   83 | 2012-01-31 08:01:00 |    6 |
+------+------+---------------------+------+

Answer 1

Try this query –

SELECT t1.rv, t1.mv, t1.datetime, t1.rawv FROM mytable t1
  JOIN (SELECT rv, mv, MAX(datetime) datetime FROM mytable GROUP BY rv, mv) t2
    ON t1.rv = t2.rv AND t1.mv = t2.mv AND t1.datetime = t2.datetime