GROUP BY ordering - mysql

Question

I have the following query in mysql:
select territory_id, platform_type_id, p.store_url
from main_itemmaster m
inner join main_iteminstance i on m.id=i.master_id
inner join main_territorypricing p on p.item_id=i.id
inner join main_territorypricingavail a on a.tp_id=p.id
where imdb_url = 'http://imdb.com/title/tt1576422/'
group by platform_type_id
Which gives me the following:
territory_id platform_type_id store_url
US Amazon http://www.amazon.com/dp/B00EQIHJAG
PT ITUNES https://itunes.apple.com/pt/movie/id582142080
However, I want to do a GROUP BY to return the territory_id="US" first if that exists. How would I do that?
This is one way I tried which looks quite dirty but does work in the version of mysql I'm using:
select * from
(select territory_id, platform_type_id, p.store_url from main_itemmaster m
inner join main_iteminstance i on m.id=i.master_id
inner join main_territorypricing p on p.item_id=i.id
inner join main_territorypricingavail a on a.tp_id=p.id
where imdb_url = 'http://imdb.com/title/tt1576422/'
order by territory_id='us' desc
) x group by platform_type_id
Which gives:
territory_id platform_type_id store_url
US Amazon http://www.amazon.com/dp/B00EQIHJAG
US ITUNES https://itunes.apple.com/us/movie/id582142080
Which is the correct result set I'm looking to get.
Here is a link to a SQL fiddle. I condensed all the data into one table to focus on the GROUP BY statement: http://sqlfiddle.com/#!9/81c3b6/2/0

Answer 1

So from the comments and the addition of the SqlFiddle it actually seems like you want to create a partitioned row number with a precedence on US per platform and then select the first record. One way of doing partitioned Row Numbers in mysql is to use variables here is an example:
SELECT
territory_id
,platform_type_id
,store_url
FROM
( SELECT
*
,#PlatFormRowNum:= IF(#prevplatform = platform_type_id, #PlatFormRowNum + 1, 1) as PlatformRowNum
,#prevplatform:= platform_type_id
FROM
main_itemmaster m
CROSS JOIN (SELECT #prevplatform:='',#PlatFormRowNum=0) var
ORDER BY
platform_type_id
,CASE WHEN territory_id = 'US' THEN 0 ELSE 1 END
,territory_id
) t
WHERE
t.PlatformRowNum = 1
ORDER BY
t.platform_type_id
SQL Fiddle: http://sqlfiddle.com/#!9/81c3b6/12
Basically this partitions the row number by platform, orders US before any other territory and then selects the first row foreach platform. The one question/trick is how do you choose which to return when US is not available for the platform simply the ascending alphabetical order of the territory_id?