Comparing all data to the data of specifically selected user ID? - mysql

Question

I have a mysql table that holds data for team games.
Objective:
Count the number of times other SquadID's have have shared the same Team value as SquadID=21
// Selections table
+--------+---------+------+
| GameID | SquadID | Team |
+--------+---------+------+
| 1 | 5 | A |
| 1 | 7 | B |
| 1 | 11 | A |
| 1 | 21 | A |
| 2 | 5 | A |
| 2 | 7 | B |
| 2 | 11 | A |
| 2 | 21 | A |
| 3 | 5 | A |
| 3 | 7 | B |
| 3 | 11 | A |
| 3 | 21 | A |
| 4 | 5 | A |
| 4 | 11 | B |
| 4 | 21 | A |
| 5 | 5 | A |
| 5 | 11 | B |
| 5 | 21 | A |
| 6 | 5 | A |
| 6 | 11 | B |
| 6 | 21 | A |
+--------+---------+------+
// Desired Result
+---------+----------+
| SquadID | TeamMate |
+---------+----------+
| 5 | 6 |
| 7 | 0 |
| 11 | 3 |
| 21 | 6 |
+----------+---------+
I've attempted to use a subquery specifying the specific player I wish to compare with and because this subquery has multiple rows, I've used in instead of =.
// Current Query
SELECT
SquadID,
COUNT(Team IN (SELECT Team FROM selections WHERE SquadID=21) AND GameID IN (SELECT GameID FROM selections WHERE SquadID=21)) AS TeamMate
FROM
selections
GROUP BY
SquadID;
The result I'm getting is the number of Games a user has played rather than the number of games a user has been on the same team as SquadID=21
// Current Result
+---------+----------+
| SquadID | TeamMate |
+---------+----------+
| 5 | 6 |
| 7 | 3 |
| 11 | 6 |
| 21 | 6 |
+---------+----------+
What am I missing?
// DESCRIBE selections;
+---------+---------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------+---------+------+-----+---------+-------+
| GameID | int(11) | NO | PRI | 0 | |
| SquadID | int(4) | NO | PRI | NULL | |
| Team | char(1) | NO | | NULL | |
| TeamID | int(11) | NO | | 1 | |
+---------+---------+------+-----+---------+-------+

Answer 1

General rule is to avoid nested selects and look for a better way of logically arranging joins. Lets look at a cross join:
From selections s1
inner join selects s2 on s1.gameid = s2.gameid and s1.team = s2.team
This will produce a cross joined list of each squadID that participated with another squadID (IE: they were in the same game and on same team). We are only interested in the times where the squad participated with squad 21, so add a where clause:
where s2.squadid = 21
Then it's simply choosing the field/count you want:
select s1.squad, count(1) as teammate
any aggregate needs a group by
group by s1.squad
Combine it together and give a go. Oddly, this will produce a list where squad 21 will be showing as playing on it's own team all 6 times. Adding a where clause can eliminate this
where s1.squadid <> s2.squadid

Answer 2

SELECT SquadID, count(t1.team) as TeamMate
FROM selections as t1 join
(select distinct team, gameid from selections where SquadID=21) as t2
on t1.Team=t2.Team and t1.Gameid=t2.Gameid
GROUP BY SquadID