如何使用数据引用的*_id字段分离/格式化string_agg()聚合数据

Question

这个问题是我之前的问题的延续，可以在这里找到。

这个SQLFiddle正在使用数据库结构&我在下面描述的查询

数据库如下:

CREATE TABLE artistnames (
    artistname_id       SERIAL  PRIMARY KEY,
    artistname          TEXT    UNIQUE NOT NULL
);
CREATE TABLE artistalias (
    artistalias_id      SERIAL  PRIMARY KEY,
    artistname_id       SERIAL  REFERENCES artistnames (artistname_id),
    artistalias         TEXT    UNIQUE NOT NULL
);
CREATE TABLE songs (
    song_id             SERIAL  PRIMARY KEY,
    song                TEXT    NOT NULL,
    artistalias_id      SERIAL  REFERENCES artistalias (artistalias_id)
);

• 一个艺术家(表artistnames)可以有0个，一个或多个别名
• 一个别名(表artistalias)只属于一个艺术家
• 一首歌曲(表格歌曲)有一个或多个艺术家(指他用来演唱这首歌的艺术家的别名)

的例子:艺术家Francis Veigar也使用别名Francis Fat和Francis Fighter。其中一首歌曲1以艺人名Francis Veigar发行，另一首歌曲2使用笔名Francis Fat，第三首歌曲3使用别名Francis Fighter，与另一位名为Peeka Boo。

使用查询

SELECT
  string_agg(distinct(artistname), ' & ') AS artist_primary_name,
  string_agg(distinct(a1.artistalias), ' & ') AS performed_song_with_alias, 
  string_agg(a2.artistalias, ' & ') AS other_pseudonymes,
  song
FROM
  artistalias a1
  left JOIN artistalias a2 ON a2.artistname_id = a1.artistname_id
  left JOIN songs s ON s.artistalias_id = a1.artistalias_id
  left JOIN artistnames ON artistnames.artistname_id = a1.artistname_id
GROUP BY song;

显示other_pseudonymes列，如

+----------------------------+-----------------------------+--------------------------------------------------------------------+--------+
| artist_primary_name        | performed_song_with_alias   | other_pseudonymes                                                  | song   |
+----------------------------+-----------------------------+--------------------------------------------------------------------+--------+
| Francis Veigar             | Francis Veigar              | Francis Veigar & Francis Fat & Francis Fighter                     | Song 1 |
+----------------------------+-----------------------------+--------------------------------------------------------------------+--------+
| Francis Veigar             | Francis Fat                 | Francis Veigar & Francis Fat & Francis Fighter                     | Song 2 |
+----------------------------+-----------------------------+--------------------------------------------------------------------+--------+
| Francis Veigar & Peeka Boo | Francis Fighter & Peeka Boo | Francis Veigar & Francis Fat & Francis Fighter & Peeka Boo & Peeka | Song 3 |
+----------------------------+-----------------------------+--------------------------------------------------------------------+--------+
| Peeka Boo                  | Peeka                       | Peeka Boo & Peeka                                                  | Song 4 |
+----------------------------+-----------------------------+--------------------------------------------------------------------+--------+

我希望它看起来像

+----------------------------+-----------------------------+--------------------------------------------------------------------+--------+
| artist_primary_name        | performed_song_with_alias   | other_pseudonymes                                                  | song   |
+----------------------------+-----------------------------+--------------------------------------------------------------------+--------+
| Francis Veigar             | Francis Veigar              | Francis Veigar & Francis Fat & Francis Fighter                     | Song 1 |
+----------------------------+-----------------------------+--------------------------------------------------------------------+--------+
| Francis Veigar             | Francis Fat                 | Francis Veigar & Francis Fat & Francis Fighter                     | Song 2 |
+----------------------------+-----------------------------+--------------------------------------------------------------------+--------+
| Francis Veigar & Peeka Boo | Francis Fighter & Peeka Boo | Francis Veigar & Francis Fat & Francis Fighter / Peeka Boo & Peeka | Song 3 |
+----------------------------+-----------------------------+--------------------------------------------------------------------+--------+
| Peeka Boo                  | Peeka                       | Peeka Boo                                                          | Song 4 |
+----------------------------+-----------------------------+--------------------------------------------------------------------+--------+

用'/'分隔两个不同艺术家使用的假名/别名。要实现这一点，查询中需要更改什么?

Answer 1

使用子查询，然后SELECT聚合元素，再使用string_agg:

SELECT
  string_agg(distinct(artistname), ' & ') AS artist_primary_name,
  string_agg(distinct(a1.artistalias), ' & ') AS performed_song_with_alias, 
  string_agg(distinct(col),' / ') AS other_pseudonymes,
  song
FROM
  artistalias a1
  left JOIN artistalias a2 ON a2.artistname_id = a1.artistname_id
  left JOIN songs s ON s.artistalias_id = a1.artistalias_id
  left JOIN artistnames ON artistnames.artistname_id = a1.artistname_id
  left join 
(SELECT 
    string_agg(a2.artistalias, ' & ') as col,
    artistname_id 
 FROM artistalias a2
 GROUP BY artistname_id)
AS aggregated_aliases ON aggregated_aliases.artistname_id = artistnames.artistname_id
GROUP BY song;