On Feb 4, 3:34 pm, "Paul Lautman" <paul.laut...@btinternet.com> wrote:
> PWX wrote:
> > I'm a somewhat educated newbie in advanced MySQL queries.
>
> > I'm attempting to create a variety of "top 10" lists related to a
> > database of meteorology information. This database records sensor data
> > every five minutes, and has been doing so since 2000. For the sake of
> > this post, let's assume that my database has only two fields: recdate
> > -- bigint(12) -- and temp -- float(7,3). The recdate format is
> > YYYYMMDDhhmm.
>
> > Creating a query to find the maximum temperature since the inception
> > of the database proved pretty easy:
>
> > SELECT recdate, temp FROM wxtable WHERE recdate BETWEEN 200001010000
> > AND 200702042355 ORDER BY temp DESC LIMIT 1
>
> > Result:
> > 200608011630 96.8
>
> > But when creating a TOP 10 list of highest daily temperatures over the
> > period was more problematic:
>
> > SELECT temp FROM wxtable WHERE recdate BETWEEN 200001010000 AND
> > 200702042355 ORDER BY temp DESC LIMIT 10
>
> > Result:
> > 200608011630 96.8
> > 200608011720 96.8
> > 200608011625 96.6
> > 200608011635 96.6
> > 200608011650 96.6
> > 200608011655 96.6
> > 200608011700 96.6
> > 200608011715 96.6
> > 200607161455 96.6
> > 200607161450 96.6
>
> > Because the weather data is recorded every five minutes, most of my
> > results came from the readings around the time of the all-time max
> > reading above. So I did get the top 10 readings, but not limited to
> > one per day over the specified range.
>
> > My question is:
> > Based on the database's use of a 12-digit integer for recording the
> > date, how can I limit my results to display only one maximum
> > temperature for each 24-hour period and therefore get a true top 10
> > list of daily temperatures.
>
> > Thanks...
>
> Use MAX() and GROUP BY utilising only the date portion of recdate.

Thanks for the suggestion. Here's what I came up with. I got
unpredictable results with this new query:

SELECT MAX(temp), recdate FROM wxtable GROUP BY SUBSTR(recdate,1,8)
ORDER BY temp DESC LIMIT 10

Result:
96.8 200608010000
94.9 200608020000
85.9 200608030000
91.6 200507250000
90.3 200307040000
94.3 200506250000
87.4 200506280000
94.5 200207020000
92.9 200607170000
86.7 200208020000

I ran this query a second time without LIMIT, and while I did appear
to get one unique high temp for each day, the sort was haphazard. The
one at the top is indeed the hottest. But the others in the top 10
shown here are not the hottest. So I think I'm closing in but
something about my sort isn't solid. Anything pop out any anyone?