pandasql：计算成对出现

Question

我试图计算A和B参加过的比赛次数， 数据集如下所示：

所以第1队和第29队参加的比赛数是2，因为它们分别是HomeTeam和AwayTeam，但是，使用我的查询，我只能将其算为一个：

SELECT HomeTeamID, AwayTeamID, Count(*) AS num_matches
FROM games GROUP BY HomeTeamID, AwayTeamID

我知道我的问题在哪里，但不知道如何解决。

Answer 1

按共同的顺序安排团队，以便分组对他们是在家还是不在不敏感。

SELECT GREATEST(HomeTeamID, AwayTeamID) AS team1, LEAST(HomeTeamID, AwayTeamID) AS team2, COUNT(*) as num_matches
FROM games
GROUP BY team1, team2

Answer 2

修订后的答案

假设您有以下数据：

然后，您可以简单地将所有团队的列表与所有游戏的列表一起加入：

SELECT
    teams.TeamID,
    CASE WHEN teams.TeamID = games.HomeTeamID THEN games.AwayTeamID ELSE games.HomeTeamID END AS OtherTeamID,
    COUNT(*) AS GamesBetween
FROM (
    SELECT HomeTeamID AS TeamID FROM games
    UNION
    SELECT AwayTeamID FROM games
) AS teams
INNER JOIN games ON teams.TeamID = games.HomeTeamID OR teams.TeamID = games.AwayTeamID
GROUP BY
    teams.TeamID,
    CASE WHEN teams.TeamID = games.HomeTeamID THEN games.AwayTeamID ELSE games.HomeTeamID END

要获得如下结果：

或者这（留作练习）：