在mySql中获得最大值的时间

Question

Table

+-------------------+------+
| Time              | value| 
+-------------------+------+
| 2011-2-1 1:01:00  | 10.1 |
+-------------------+------+
| 2011-2-1 1:03:15  | 7.7  |
+-------------------+------+
| 2011-2-1 1:05:21  | 5.4  |
+-------------------+------+
| 2011-2-1 1:06:00  | 2.3  |
+-------------------+------+
| 2011-2-1 1:09:30  | 4.2  |
+-------------------+------+
| 2011-2-1 1:11:10  | 6.2  |
+-------------------+------+

I want to get the maximum value for every 5 minutes. For example,

• In the first 5 minutes, the maximum value is 10.1

• In the second 5 minutes, the max value is 4.2

In additional, I would also like to get the corresponding 'Time' for each maximum value.

Many thanks

Answer 1

Your select statement needs to group all time by 5 minute intervals and then have a max() on value.

This talks about grouping time in MySQL:

http://forums.mysql.com/read.php?20,131939,131955#msg-131955

To do the five minute grouping, use the round() function in combination with unix_timestamp()

For five minutes:

round(unix_timestamp([time_field]) / 300)

So all together, your select might look like this (untested):

select
     round(unix_timestamp([time_field]) / 300) as Time,
     max(value) as Value
from
     [table_name]
group by
     round(unix_timestamp([time_field]) / 300)

Answer 2

A few different ways of doing this.

One that should work most of the time:

SELECT `Time`,`value`
FROM (SELECT `Time`,`value` FROM table ORDER BY `value` DESC) t2
GROUP BY UNIX_TIMESTAMP(`Time`) DIV 300

This relies on the fact that MySQL usually picks the first row it finds and returns that in light of the group by. So the inner query that first orders it by value, ensures the largest value is always found first. (But its not totally reliable, as the mysql optimizer could end up arranging the query differently. Its not actully defined which row should be returned.

If that doesn't work for you, try searching for the Max-group-concat trick.