如何使用 Postgres json_agg 组合多个一对多结果？

Question

函数中存在多个 json_aggregate

我的 Postgres 数据库的主表与其他表有几个一对多的关系，每个关系都由一个[*]_references表连接。我需要一个结果，该结果与其他表的结果一起以 JSON 形式返回。我可以加入以获得我想要的数据，如下所示：

SELECT ft.id, json_agg(genres) AS genres 
FROM ft_references ft 
INNER JOIN genre_references gr ON gr.reference_id = ft.id 
INNER JOIN genres_catalog genres ON gr.genre_id = genres.id 
WHERE ft.id = 2 GROUP BY ft.id;

 id |                      genres                      
----+--------------------------------------------------
  2 | [{"id":1,"name":"Action","other_info":null},    +
    |  {"id":2,"name":"Adventure","other_info":null}, +
    |  {"id":5,"name":"Comedy","other_info":null},    +
    |  {"id":8,"name":"Drama","other_info":null},     +
    |  {"id":10,"name":"Fantasy","other_info":null},  +
    |  {"id":13,"name":"Horror","other_info":null},   +
    |  {"id":25,"name":"War","other_info":null}]
(1 row)

同样，我的第二组结果：

SELECT ft.id, json_agg(tales) AS tales 
FROM ft_references ft 
INNER JOIN tale_references tr ON tr.reference_id = ft.id 
INNER JOIN tale_catalog tales ON tales.id = tr.tale_catalog_id 
WHERE ft.id = 2 GROUP BY ft.id;

 id |                                      tales                                      
----+---------------------------------------------------------------------------------
  2 | [{"id":8,"tale_id":"124","tale_type":"The Three Brothers","other_info":null},  +
    |  {"id":39,"tale_id":"327A","tale_type":"Hansel and Gretel","other_info":null}, +
    |  {"id":82,"tale_id":"570","tale_type":"Pied Piper","other_info":null}]
(1 row)

这两个结果都是正确的。然而，当我想把它们放在一起时，突然我得到了我的类型和故事结果的乘积（即：三重结果！）：

SELECT ft.id, json_agg(genres) AS genres, json_agg(tale) AS tales 
FROM ft_references ft 
INNER JOIN genre_references gr ON gr.reference_id = ft.id 
INNER JOIN genres_catalog genres ON gr.genre_id = genres.id 
INNER JOIN tale_references tr ON tr.reference_id = ft.id 
INNER JOIN tale_catalog tale ON tale.id = tr.tale_catalog_id 
WHERE ft.id = 2 GROUP BY ft.id, gr.reference_id, tr.reference_id;
 id |                      genres                      |                                      tales                                      
----+--------------------------------------------------+---------------------------------------------------------------------------------
  2 | [{"id":1,"name":"Action","other_info":null},    +| [{"id":82,"tale_id":"570","tale_type":"Pied Piper","other_info":null},         +
    |  {"id":1,"name":"Action","other_info":null},    +|  {"id":39,"tale_id":"327A","tale_type":"Hansel and Gretel","other_info":null}, +
    |  {"id":1,"name":"Action","other_info":null},    +|  {"id":8,"tale_id":"124","tale_type":"The Three Brothers","other_info":null},  +
    |  {"id":2,"name":"Adventure","other_info":null}, +|  {"id":82,"tale_id":"570","tale_type":"Pied Piper","other_info":null},         +
    |  {"id":2,"name":"Adventure","other_info":null}, +|  {"id":39,"tale_id":"327A","tale_type":"Hansel and Gretel","other_info":null}, +
    |  {"id":2,"name":"Adventure","other_info":null}, +|  {"id":8,"tale_id":"124","tale_type":"The Three Brothers","other_info":null},  +
    |  {"id":5,"name":"Comedy","other_info":null},    +|  {"id":82,"tale_id":"570","tale_type":"Pied Piper","other_info":null},         +
    |  {"id":5,"name":"Comedy","other_info":null},    +|  {"id":39,"tale_id":"327A","tale_type":"Hansel and Gretel","other_info":null}, +
    |  {"id":5,"name":"Comedy","other_info":null},    +|  {"id":8,"tale_id":"124","tale_type":"The Three Brothers","other_info":null},  +
    |  {"id":8,"name":"Drama","other_info":null},     +|  {"id":82,"tale_id":"570","tale_type":"Pied Piper","other_info":null},         +
    |  {"id":8,"name":"Drama","other_info":null},     +|  {"id":39,"tale_id":"327A","tale_type":"Hansel and Gretel","other_info":null}, +
    |  {"id":8,"name":"Drama","other_info":null},     +|  {"id":8,"tale_id":"124","tale_type":"The Three Brothers","other_info":null},  +
    |  {"id":10,"name":"Fantasy","other_info":null},  +|  {"id":82,"tale_id":"570","tale_type":"Pied Piper","other_info":null},         +
    |  {"id":10,"name":"Fantasy","other_info":null},  +|  {"id":39,"tale_id":"327A","tale_type":"Hansel and Gretel","other_info":null}, +
    |  {"id":10,"name":"Fantasy","other_info":null},  +|  {"id":8,"tale_id":"124","tale_type":"The Three Brothers","other_info":null},  +
    |  {"id":13,"name":"Horror","other_info":null},   +|  {"id":82,"tale_id":"570","tale_type":"Pied Piper","other_info":null},         +
    |  {"id":13,"name":"Horror","other_info":null},   +|  {"id":39,"tale_id":"327A","tale_type":"Hansel and Gretel","other_info":null}, +
    |  {"id":13,"name":"Horror","other_info":null},   +|  {"id":8,"tale_id":"124","tale_type":"The Three Brothers","other_info":null},  +
    |  {"id":25,"name":"War","other_info":null},      +|  {"id":82,"tale_id":"570","tale_type":"Pied Piper","other_info":null},         +
    |  {"id":25,"name":"War","other_info":null},      +|  {"id":39,"tale_id":"327A","tale_type":"Hansel and Gretel","other_info":null}, +
    |  {"id":25,"name":"War","other_info":null}]       |  {"id":8,"tale_id":"124","tale_type":"The Three Brothers","other_info":null}]
(1 row)

如何重构我的查询，以便我只获得第一个示例中的正确“流派”结果以及第二个示例中的正确“故事”结果，而不是像我的实际情况那样过度完成（第三）例子？

Answer 1

答案如下：

SELECT ft.id, 
        json_agg(DISTINCT genres.*) AS genres, 
        json_agg(DISTINCT tale.*) AS tales
  FROM ft_references ft 
  JOIN genre_references gr ON gr.reference_id = ft.id
  JOIN genres_catalog genres ON genres.id = gr.genre_id
  JOIN tale_references tr ON tr.reference_id = ft.id
  JOIN tale_catalog tale ON tale.id = tr.tale_catalog_id
  WHERE ft.id = 2 GROUP BY ft.id;