sql - 如何将多列分组为单个数组或类似数组？

Question

我希望我的查询返回结构如下的结果，其中 tags 是数组数组或类似数组:

id | name | tags
1    a      [[1, "name1", "color1"], [2, "name2", color2"]]
2    b      [[1, "name1", "color1"), (3, "name3", color3"]]

我希望这个查询能够工作，但它给了我一个错误:

SELECT  i.id, i.name, array_agg(t.tag_ids, t.tag_names, t.tag_colors) as tags 
FROM    ITEMS
LEFT OUTER JOIN (
  SELECT      trm.target_record_id
            , array_agg(tag_id) as tag_ids
            , array_agg(t.tag_name) as tag_names
            , array_agg(t.tag_color) as tag_colors
  FROM        tags_record_maps trm
  INNER JOIN  tags t on t.id = trm.tag_id
  GROUP BY    trm.target_record_id
) t on t.target_record_id = i.id;

错误:

PG::UndefinedFunction: ERROR:  function array_agg(integer[], character varying[], character varying[]) does not exist
LINE 1: ..., action_c2, action_c3, action_name, action_desc, array_agg(...
HINT:  No function matches the given name and argument types. You might need to add explicit type casts.

这个查询有效并产生类似的结果(但不是我想要的):

SELECT  i.id, i.name, t.tag_ids, t.tag_names, t.tag_colors as tags  as tags 
FROM    ITEMS
LEFT OUTER JOIN (
  SELECT      trm.target_record_id, array_agg(tag_id) as tag_ids, array_agg(t.tag_name) as tag_names, array_agg(t.tag_color) as tag_colors
  FROM        tags_record_maps trm
  INNER JOIN  tags t on t.id = trm.tag_id
  GROUP BY    trm.target_record_id
) t on t.target_record_id = i.id;

结果:

id | name | tag_ids | tag_names         | tag_colors          
1    a      [1, 2]    ["name1, "name2"]   ["color1", "color2"]
1    a      [1, 3]    ["name1, "name3"]   ["color1", "color3"]

编辑:

此查询几乎生成了我正在寻找的内容，除了它命名为 json 键 f1、f2、f3。如果我能将它们命名为 id、name、color 就完美了:

  SELECT        trm.target_record_id, json_agg( (t.id, t.tag_name, t.tag_color) )
  FROM          tags_record_maps trm
  INNER JOIN    tags t on t.site_id = trm.site_id and t.id = trm.tag_id
  GROUP BY      trm.target_record_id
  having count(*) > 1;

结果:

[{"f1":1,"f2":"name1","f3":"color1"},{"f1":2,"f2":"name2","f3":"color2"}]

Answer 1

(t.id, t.tag_name, t.tag_color) 是 ROW(t.id, t.tag_name, t.tag_color) 的简短语法 - 和一个ROW constructor不保留嵌套的属性名称。 The manual:

By default, the value created by a ROW expression is of an anonymous record type. If necessary, it can be cast to a named composite type —either the row type of a table, or a composite type created withCREATE TYPE AS.

大胆强调我的。要在结果中也获得正确的键名，请按照引用中的建议转换到已注册的复合类型，使用嵌套子选择，或简单地使用json_build_object()在 Postgres 9.4 或更新版本中(有效地避免先验的 ROW 构造函数):

SELECT trm.target_record_id
     , <b>json_agg(json_build_object('id', t.id
                                , 'tag_name', t.tag_name
                                , 'tag_color', t.tag_color)) AS tags</b>
FROM   tags_record_maps trm
JOIN   tags             t USING (site_id)
WHERE  t.id = trm.tag_id
GROUP  BY trm.target_record_id
HAVING count(*) > 1;

我使用原始的列名，但您可以自由选择您的键名。在你的情况下:

       json_agg(json_build_object('id', t.id
                                , 'name', t.tag_name
                                , 'color', t.tag_color)) AS tags

详细解释:

• Return multiple columns of the same row as JSON array of objects