如何使用 postgresql 获取数组子查询？答案

【问题标题】：How to get an array subquery using postgress sql?如何使用 postgresql 获取数组子查询？
【发布时间】：2020-01-06 15:39:02
【问题描述】：

所以，我需要的是以下 json。

var plan = [{
  id: 11,
  title: 'give a title',
  actions: [
   {id: 1,
   planId: 11,
   title: 'give action name'},
   {
     id: 3,
     planId: 11,
     title: 'give another action name'
   }
  ]},
{
      id: 13,
      title: 'thirteen a title',
      actions: [
       {id: 1,
       planId: 13,
       title: 'thirteen action name'},
       {
         id: 3,
         planId: 13,
         title: 'thirteen another action name'
       }
      ]}
]

所以我有 2 个表格、计划和行动。两个表之间的关系是计划有很多动作。计划（id，标题） Action(id, title, planId)

SELECT
*,
ARRAY (
    SELECT
        jsonb_build_object ('id',
            m.id,
            'title',
            m.title)
    FROM
        actions a
        INNER JOIN plan p ON p.id = a.planid
    ) AS actions
FROM
    plan

我不确定如何获得每个计划下的相关操作。

【问题讨论】：

标签： sql arrays json postgresql

【解决方案1】：

这些查询可能是您正在寻找的：

数据样本

CREATE TEMPORARY TABLE plan (id INT, title TEXT);
CREATE TEMPORARY TABLE action (id INT, title TEXT, planid INT);

INSERT INTO plan VALUES (1,'plan a'),(2,'plan b');
INSERT INTO action VALUES (1,'1st action plan a',1),
                          (2,'2nd action plan a',1),
                          (3,'1st action plan b',2);

查询 - 多条 json 记录

SELECT 
  json_build_object(
    'id',p.id,'title',p.title,
    'actions',(SELECT json_agg(row_to_json(t)) 
               FROM (SELECT id,title,planid 
               FROM action WHERE planid = p.id) t)) AS myjson
FROM plan p;
                                                                        myjson                                                                     
------------------------------------------------------------------------------------------------------------------------------------------------
 {"id" : 1, "title" : "plan a", "actions" : [{"id":1,"title":"1st action plan a","planid":1}, {"id":2,"title":"2nd action plan a","planid":1}]}
 {"id" : 2, "title" : "plan b", "actions" : [{"id":3,"title":"1st action plan b","planid":2}]}
(2 Zeilen)

查询 - 单个 json 记录

SELECT json_agg(row_to_json(myjson)) FROM 
(SELECT 
  json_build_object(
    'id',p.id,'title',p.title,
    'actions',(SELECT json_agg(row_to_json(t)) 
               FROM (SELECT id,title,planid 
               FROM action WHERE planid = p.id) t)) as plan
FROM plan p) myjson;

-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 [{"plan":{"id" : 1, "title" : "plan a", "actions" : [{"id":1,"title":"1st action plan a","planid":1}, {"id":2,"title":"2nd action plan a","planid":1}]}}, {"plan":{"id" : 2, "title" : "plan b", "actions" : [{"id":3,"title":"1st action plan b","planid":2}]}}]
(1 Zeile)

【讨论】：

IMO array_to_json(array_agg(...)) 可以简化为 json_agg(...)
@Abelisto 你说得对……刚刚编辑了我的答案。谢谢！

【解决方案2】：

应该这样做：

SELECT
array_to_json(array_agg(row_to_json(tb_data))) AS data FROM 
(
    SELECT
    tb_plan.id,
    tb_plan.titile,
    array_to_json(array_agg(row_to_json(tb_action))) AS actions
    FROM plan tb_plan
    INNER JOIN "action" tb_action ON tb_action.id = tb_action.id
    GROUP BY 1,2
) tb_data

Here's the documentation 关于这个查询中使用的 postgresql 的 json 函数。

【讨论】：

【解决方案3】：

这样的事情应该可以完成：

SELECT json_agg(data)
FROM
  (SELECT p.id,
          p.title,
          json_agg(a) FILTER (WHERE a.plan_id IS NOT NULL) AS actions
   FROM PLAN p
   LEFT JOIN action a ON a.plan_id = p.id
   GROUP BY p.id,
            p.title) data;

[
  {
    "id": 1,
    "title": "test plan 1",
    "actions": [
      {
        "id": 1,
        "title": "test action 1",
        "plan_id": 1
      },
      {
        "id": 2,
        "title": "test action 2",
        "plan_id": 1
      }
    ]
  },
  {
    "id": 2,
    "title": "test plan 2",
    "actions": [
      {
        "id": 3,
        "title": "test action 3",
        "plan_id": 2
      },
      {
        "id": 4,
        "title": "test action 4",
        "plan_id": 2
      }
    ]
  }
]

【讨论】：