MySQL Order by before group by in view (No Sub-Querys) - mysql

Question

I realize this question has been asked quite a few times, however i haven't managed to find a working solution for my case.
Essentially my problem arises because MySQL Doesn't allow sub-querys in views.
I found a few workarounds but they don't seem to work.
In more detail...
My first table (competitions) stores a users competitions:
id_tournament | id_competition | id_user | result
-------------------------------------------------
1 | 1 | 1 | 10
1 | 1 | 2 | 30
1 | 2 | 1 | 20
1 | 2 | 3 | 50
1 | 3 | 2 | 90
1 | 3 | 3 | 100
1 | 3 | 4 | 85
In this example there are three competitions:
(
user1 vs. user2,
user1 vs. user3,
user2 vs. user3 vs. user4
)
My problem is that i need to define a view that gives me the winners in each competition.
Expected Result:
id_tournament | id_competition | id_winner
------------------------------------------
1 | 1 | 2
1 | 2 | 3
1 | 3 | 3
This can be solved with the query:
SELECT
id_tournament,
id_competition,
id_user as id_winner
FROM (
SELECT * FROM competitions ORDER BY result DESC
) x GROUP BY id_tournament, id_competition
This query however uses a subquery (not allowed in views), so my first solution was to define a 'helper view'as :
CREATE VIEW competitions_helper AS (
SELECT * FROM competitions ORDER BY result DESC
);
CREATE VIEW competition_winners AS (
SELECT
id_tournament,
id as id_competition,
id_user as winner
FROM competitions_helper GROUP BY id_tournament, id_competition
);
However this does not seem to give the correct result.
It's result will then be:
id_tournament | id_competition | id_winner
------------------------------------------
1 | 1 | 1
1 | 2 | 1
1 | 3 | 1
What i don't understand is why it works when i use Sub-querys and why it gives a different result with the exact same statement in a view.
Any help is appreciated, thanks alot.

Answer 1

This is due to the GROUP BY behaviour.
In this case, the server is free to choose any value from each group, so unless they are the same, the values chosen are indeterminate, which is probably not what you want.
I would solve the problem in this way:
CREATE VIEW competitions_helper AS (
SELECT id_tournament,
id_competition,
MAX(result) as winning_result
FROM competitions
GROUP BY id_tournament,
id_competition
);
CREATE VIEW competition_winners AS (
SELECT c.id_tournament,
c.id_competition,
c.id_user
FROM competitions c
INNER JOIN competitions_helper ch
ON ch.id_tournament = c.id_tournament
AND ch.id_competition = c.id_competition
AND ch.winning_result = c.result
);