Group by various column (with various joins) but sum distinct other column - mysql

Question

I have to do some reporting, involving various tables, and having couple of SUMs, COUNTs, etc and everything is OK. But the last thing I have to resolve is SUM by another which is not in the grouped columns.
I'll give you an example (stripped down from what I have) so you can understand the tongue-twister in the previous paragraph.
Suppose I have a query with a couple of joins that get me this result, or a temporary table, or whatever:
(this is a trimmed down version, in the original I have much more columns and groupbys)
APP_ID CAT_ID CAT_DESCRIP APP_START APP_END DETAIL_ID DET_QTY DETAIL_PRICE
1 1 Categ One 900 960 1 10 150.00
1 1 Categ One 900 960 2 8 20.00
1 1 Categ One 900 960 3 12 30.00
1 1 Categ One 900 960 4 5 100.00
2 2 Categ Two 600 720 5 12 150.00
2 2 Categ Two 600 720 6 10 50.00
3 2 Categ Two 1200 1260 7 5 20.00
I need to get something like this: (the bolded column is the important)
SELECT
CAT_ID,
CAT_DESCRIP,
SUM(DET_QTY) as TotalQTY,
SUM(DETAIL_PRICE) as TotalPrice,
COUNT(DISTINCT APP_ID) as CountOfApps,
(GET THE SUM OF (APP_END - APP_START) ONLY ONE TIME BY APP_ID INTO THIS CATEG) as TimeInMinutesByCategory
FROM
MyTable
GROUP BY
CAT_ID
And the result has to give me this:
CAT_ID CAT_DESCRIP TotalQTY TotalPrice CountOfApps TimeInMinutesByCategory
1 Categ One 35 300.00 1 60
2 Categ Two 27 220.00 2 180
Thanks for your help!

Answer 1

I think this will do the job... or if not, a little tweaking on the sytnax for max(app_start) - max(app_end) should do the job
The idea is, summarize the data in a subquery by app_id and cat_id. Select the max value of start and end, grouped by app_id and cat_id. Since there will only be one value per each distinct pair of app_id and cat_id, we're essentially just deduping.
Then, join the subquery to the main query and summarize by category id.
SELECT
a.CAT_ID,
a.CAT_DESCRIP,
SUM(a.DET_QTY) as TotalQTY,
SUM(a.DETAIL_PRICE) as TotalPrice,
COUNT(DISTINCT a.APP_ID) as CountOfApps,
SUM(b.TimeInMinutesByCategory) AS TimeInMinutesByCategory
FROM
MyTable AS a
INNER JOIN (
SELECT APP_ID, CAT_ID, max(app_start) - max(app_end) AS TimeInMinutesByCategory
FROM MyTable
GROUP BY APP_ID, CAT_ID) AS b
ON a.cat_id = b.cat_id
AND a.app_id = b.app_id
GROUP BY
a.CAT_ID