SQL query, how to improve? - mysql

Question

I did a task to write an SQL query and I wonder if I can improve it somehow.
Description:
Let's say we have a db on some online service
Let's create tables, and insert some data
create table players (
player_id integer not null unique,
group_id integer not null
);
create table matches (
match_id integer not null unique,
first_player integer not null,
second_player integer not null,
first_score integer not null,
second_score integer not null
);
insert into players values(20, 2);
insert into players values(30, 1);
insert into players values(40, 3);
insert into players values(45, 1);
insert into players values(50, 2);
insert into players values(65, 1);
insert into matches values(1, 30, 45, 10, 12);
insert into matches values(2, 20, 50, 5, 5);
insert into matches values(13, 65, 45, 10, 10);
insert into matches values(5, 30, 65, 3, 15);
insert into matches values(42, 45, 65, 8, 4);
The output of the query should be:
group_id | winner_id
--------------------
1 | 45
2 | 20
3 | 40
So, we should output the winner (player id) of each group. Winner is the player, who got max amount of points in matches.
If user is alone in the group - he's a winner automatically, in case players have equal amount of points - the winner is the one, who has lower id value.
Output should be ordered by group_id field
My solution:
SELECT
results.group_id,
results.winner_id
FROM
(
SELECT
summed.group_id,
summed.player_id AS winner_id,
MAX(summed.sum) AS total_score
FROM
(
SELECT
mapped.player_id,
mapped.group_id,
SUM(mapped.points) AS sum
FROM
(
SELECT
p.player_id,
p.group_id,
CASE WHEN p.player_id = m.first_player THEN m.first_score WHEN p.player_id = m.second_player THEN m.second_score ELSE 0 END AS points
FROM
players AS p
LEFT JOIN matches AS m ON p.player_id = m.first_player
OR p.player_id = m.second_player
) AS mapped
GROUP BY
mapped.player_id
) as summed
GROUP BY
summed.group_id
ORDER BY
summed.group_id
) AS results;
It works, but I'm 99% sure it can be cleaner. Will be thankful for any suggestions

Answer 1

First, use UNION ALL to extract from matches 2 columns: player_id and score for all players and their scores.
Then aggregate to get each player's total score.
Finally do a LEFT join of players to the resultset you obtained, use GROUP_CONCAT() to collect all players of each group in descending order respective to their total score and with SUBSTRING_INDEX() pick the 1st player:
SELECT p.group_id,
SUBSTRING_INDEX(GROUP_CONCAT(p.player_id ORDER BY t.score DESC, p.player_id), ',', 1) winner_id
FROM players p
LEFT JOIN (
SELECT player_id, SUM(score) score
FROM (
SELECT first_player player_id, first_score score FROM matches
UNION ALL
SELECT second_player, second_score FROM matches
) t
GROUP BY player_id
) t ON t.player_id = p.player_id
GROUP BY p.group_id;
See the demo.
Note that, by doing a LEFT join, you get in the results all groups, even the ones that do not have any players that participated in any match (just like your sample data), in which case the winner is an arbitrary player (just like your expected results).

Answer 2

You can unpivot the matches table and sum the points per player (which is I think what you want):
select p.player_id, p.group_id, sum(score) as sum_score
from ((select first_player as player_id, first_score as score
from matches
) union all
(select second_player as player_id, second_score as score
from matches
)
) mp
players p
using (player_id)
group by p.player_id, p.group_id;
Next, you can introduce a window function to get the top:
select player_id, group_id, sum_score
from (select p.player_id, p.group_id, sum(score) as sum_score,
row_number() over (partition by p.group_id order by sum(score) desc p.player_id asc) as seqnum
from ((select first_player as player_id, first_score as score
from matches
) union all
(select second_player as player_id, second_score as score
from matches
)
) mp
players p
using (player_id)
group by p.player_id, p.group_id
) pg
where seqnum = 1;
If you actually want the maximum score over all the matches rather than the sum(), then use max() instead of sum().

Answer 3

Here's another way:
WITH match_records AS (
SELECT match_id,first_player players, first_score scores FROM matches UNION ALL
SELECT match_id,second_player, second_score FROM matches
)
SELECT group_id, player_id
FROM
(SELECT group_id, player_id, players, SUM(scores) ts,
ROW_NUMBER() OVER (PARTITION BY group_id ORDER BY SUM(scores) DESC) pos
FROM players p LEFT JOIN match_records mr ON mr.players=p.player_id
GROUP BY group_id, player_id, players) fp
WHERE pos=1
ORDER BY group_id;
It's basically the same idea as others (to un-pivot the matches table) but with a slightly different operation.
Demo fiddle