(SQL) 为结点中的每个 ID 获取一列 table

Question

[如果这个问题已经在其他地方得到了回答，我会感激地接受 link 并删除它]

我有一个播放列表table:

pid | name 

1   | playlist 1 
2   | playlist 2
3   | playlist 3

还有一首歌table:

sid | name 

1   | song 1 
2   | song 2
3   | song 3

他们与这个路口有联系table:

pid | sid 

1   | 1 
1   | 2
1   | 3
2   | 2 
3   | 1

我已经设法让连接开始工作

SELECT      playlists.pid, songs.*
FROM        playlists
INNER JOIN  playlist_contains_song
ON          playlists.pid = playlist_contains_song.pid
INNER JOIN  songs
ON          playlist_contains_song.sid = songs.sid;

这给了我这个结果：

pid | sid | name 

1   | 1   | song 1    
1   | 2   | song 2
1   | 3   | song 3    
2   | 2   | song 2    
3   | 1   | song 1

现在我想知道是否有办法获取这些数据'horizontally'。首选输出应如下所示：

pid | 1 | 2 | 3

1   | 1 | 1 | 1
2   | 0 | 1 | 0
3   | 1 | 0 | 0

列是 sid，值代表 true 或 false，表示歌曲是否在播放列表中。

提前致谢！

Answer 1

使用数据透视查询：

SELECT pid,
       COUNT(*) FILTER (WHERE sid = 1) AS "1",
       COUNT(*) FILTER (WHERE sid = 2) AS "2",
       COUNT(*) FILTER (WHERE sid = 3) AS "3"
FROM playlist_contains_song
GROUP BY pid
ORDER BY pid;

(SQL) 为结点中的每个 ID 获取一列 table

(SQL) Getting a column for every ID out of a junction table

sql

postgresql

select

join

junction-table