SQL – Aggregate all EXCEPT group

Consider the following sample table from a soccer tournament (let’s call this table matches)

+----------+---------+--------------+
| match_id | club_id | goals_scored |
+----------+---------+--------------+
|        1 |       1 |            1 |
|        1 |       2 |            0 |
|        2 |       1 |            1 |
|        2 |       3 |            1 |
|        3 |       1 |            0 |
|        3 |       4 |            2 |
|        4 |       2 |            2 |
|        4 |       3 |            4 |
|        5 |       2 |            4 |
|        5 |       4 |            0 |
|        6 |       3 |            1 |
|        6 |       4 |            1 |
+----------+---------+--------------+

The resulting table we want should give us each club’s total goals scored AND goals conceded:

+---------+--------------+----------------+
| club_id | goals_scored | goals_conceded |
+---------+--------------+----------------+
|       1 |            2 |              4 |
|       2 |            6 |              4 |
|       3 |            6 |              4 |
|       4 |            3 |              5 |
+---------+--------------+----------------+

Getting goals scored is straight forward enough…

SELECT SUM(goals_scored),
       club_id
  FROM matches
 GROUP BY club_id

but I am absolutely flummoxed as to how to get it for each team’s opponents.

I could, of course, construct a pretty complex array of subqueries to get there. If this were application-side work I’d likely just stuff it in a loop and iterate over each club to get there, but my use case requires a SQL answer if possible. Any thoughts?

edit: also if anyone has any better ideas on how to title this question, I’m all ears – I’m not really sure exactly how to describe this problem in the first place.

Answer

We can use a self-join approach here:

SELECT
    m1.club_id,
    SUM(m1.goals_scored) AS goals_scored,
    SUM(m2.goals_scored) AS goals_conceded
FROM matches m1
INNER JOIN matches m2
    ON m2.match_id = m1.match_id AND
        m2.club_id <> m1.club_id
GROUP BY
    m1.club_id
ORDER BY
    m1.club_id;

This approach brings the goals conceded by each club to the other club, for each match, into a single row. We then just aggregate by club to get the two sums.

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Answer