如何通过链接表获取数据并将行旋转到列

Question

我正在尝试通过链接表从我的一个表中获取一些数据，并将多行输出为一列 - 这是一个个人项目，旨在帮助我学习，因为我对 SQL 还很陌生而且我什至不确定要搜索什么 - 我已经查找了我能找到的内容，但没有成功。

我拥有的是

表 1 - 播放

id	name
1	gamename1
2	gamename1
3	gamename2

表 2 - 玩家

id	name
1	playername1
2	playername1
3	playername2

链接表

play_id	players_id
1	1
1	2
1	3
2	2
3	2
3	3

我希望看到的内容（不必完全匹配，而是显示每个 play_id 的所有玩家，每行一个 play_id）

Game id	Player name1	Player name2	Player name3
gamename1	name1	name2	name3
gamename1	name2
gamename2	name2	name3

我认为这会起作用，但它没有，我能够获得每场比赛的第一个玩家（使用 group by），或者如果我删除它，我会为我的链接表中的每个条目获得一行.

SELECT
    plays.name,
    players.name
FROM
    plays
    INNER JOIN linktable
        ON linktable.play_id = play.id
    INNER JOIN players
        ON linktable.players_id = players.id
GROUP BY
    plays.id

提前感谢你们的帮助！

Answer 1

考虑从Linktable 开始，然后是LEFT JOIN（INNER JOIN，如果没有孤立链接）到查找表Plays 和Players。然后运行条件聚合以进行透视：

SELECT p.`name` AS gamername
     , CASE WHEN ps.id = 1 THEN ps.name END AS Player_name_1
     , CASE WHEN ps.id = 2 THEN ps.name END AS Player_name_2
     , CASE WHEN ps.id = 3 THEN ps.name END AS Player_name_3

FROM linktable l
LEFT JOIN plays p
   ON l.play_id = p.id
LEFT JOIN players ps
   ON l.player_id = ps.id
ORDER BY p.id;

Online Demo

gamername	Player_name_1	Player_name_2	Player_name_3
gamename1	playername1	NULL	NULL
gamename1	NULL	playername2	NULL
gamename2	NULL	NULL	playername3
gamename3	NULL	playername2	NULL
gamename3	NUL	playername2	NULL
gamename3	NULL	NULL	playername3

如果你汇总更好：

SELECT p.`name` AS gamername
     , MAX(CASE WHEN ps.id = 1 THEN ps.name END) AS Player_name_1
     , MAX(CASE WHEN ps.id = 2 THEN ps.name END) AS Player_name_2
     , MAX(CASE WHEN ps.id = 3 THEN ps.name END) AS Player_name_3

FROM linktable l
LEFT JOIN plays p
   ON l.play_id = p.id
LEFT JOIN players ps
   ON l.player_id = ps.id
GROUP BY p.`name`;

gamername	Player_name_1	Player_name_2	Player_name_3
gamename1	playername1	playername2	playername3
gamename2	NULL	playername2	NULL
gamename3	NULL	playername2	playername3