如何将具有树结构的表聚合到单个嵌套的JSON对象？

Question

我在Postgres 11.4数据库中有一个表，具有自引用树结构：

+------------+
| account    |  
+------------+
| id         |
| code       | 
| type       |
| parentId   | -- references account.id
+------------+

每个孩子可以有另一个孩子，嵌套级别没有限制。

我想从中生成一个JSON对象，嵌套所有子对象（复活）。

可以用单个查询解决这个问题吗？或者使用Typeform和一个表的任何其他解决方案<否则我将不得不在服务器端手动绑定数据。

我尝试了这个查询：

SELECT account.type, json_agg(account) as accounts
FROM account
-- LEFT JOIN "account" "child" ON "child"."parentId"="account"."id" -- tried to make one column child
GROUP BY account.type   

结果：

[
  ...
  {
    "type": "type03",
    "accounts": [
      {
        "id": 28,
        "code": "acc03.001",
        "type": "type03",
        "parentId": null
      },
      {
        "id": 29,
        "code": "acc03.001.001",
        "type": "type03",
        "parentId": 28
      },
      {
        "id": 30,
        "code": "acc03.001.002",
        "type": "type03",
        "parentId": 28
      }
    ]
  }
  ...
]

相反，我期望这样：

[
  ...
  {
    "type": "type03",
    "accounts": [
      {
        "id": 28,
        "code": "acc03.001",
        "type": "type03",
        "parentId": null,
        "child": [
          {
            "id": 29,
            "code": "acc03.001.001",
            "type": "type03",
            "parentId": 28
          },
          {
            "id": 30,
            "code": "acc03.001.002",
            "type": "type03",
            "parentId": 28
          }
        ]
      }
    ]
  }
  ...
]

Answer 1

这很棘手。这是一个递归问题，但是标准的递归CTE没有能力处理它，因为我们需要在每个级别上聚合，而CTE不允许在递归术语中聚合。

我用PL/pgSQL函数解决了它：

CREATE OR REPLACE FUNCTION f_build_jsonb_tree(_type text = NULL)
  RETURNS jsonb
  LANGUAGE plpgsql AS
$func$
DECLARE
   _nest_lvl int;

BEGIN
   -- add level of nesting recursively
   CREATE TEMP TABLE t ON COMMIT DROP AS
   WITH RECURSIVE t AS (
      SELECT *, 1 AS lvl
      FROM   account
      WHERE  "parentId" IS NULL
      AND   (type = _type OR _type IS NULL) -- default: whole table

      UNION ALL
      SELECT a.*, lvl + 1
      FROM   t
      JOIN   account a ON a."parentId" = t.id
      )
   TABLE t;
   
   -- optional idx for big tables with many levels of nesting
   -- CREATE INDEX ON t (lvl, id);

   _nest_lvl := (SELECT max(lvl) FROM t);

   -- no nesting found, return simple result
   IF _nest_lvl = 1 THEN 
      RETURN (  -- exits functions
      SELECT jsonb_agg(sub) -- AS result
      FROM  (
         SELECT type
              , jsonb_agg(sub) AS accounts
         FROM  (
            SELECT id, code, type, "parentId", NULL AS children
            FROM   t
            ORDER  BY type, id
            ) sub
         GROUP BY 1
         ) sub
      );
   END IF;

   -- start collapsing with leaves at highest level
   CREATE TEMP TABLE j ON COMMIT DROP AS
   SELECT "parentId" AS id
        , jsonb_agg (sub) AS children
   FROM  (
      SELECT id, code, type, "parentId"  -- type redundant?
      FROM   t
      WHERE  lvl = _nest_lvl
      ORDER  BY id
      ) sub
   GROUP  BY "parentId";

   -- optional idx for big tables with many levels of nesting
   -- CREATE INDEX ON j (id);

   -- iterate all the way down to lvl 2
   -- write to same table; ID is enough to identify
   WHILE _nest_lvl > 2
   LOOP
      _nest_lvl := _nest_lvl - 1;

      INSERT INTO j(id, children)
      SELECT "parentId"     -- AS id
           , jsonb_agg(sub) -- AS children
      FROM  (
         SELECT id, t.code, t.type, "parentId", j.children  -- type redundant?
         FROM   t
         LEFT   JOIN j USING (id)  -- may or may not have children
         WHERE  t.lvl = _nest_lvl
         ORDER  BY id
         ) sub
      GROUP  BY "parentId";
   END LOOP;

   -- nesting found, return nested result
   RETURN ( -- exits functions
   SELECT jsonb_agg(sub) -- AS result
   FROM  (
      SELECT type
           , jsonb_agg (sub) AS accounts
      FROM  (
         SELECT id, code, type, "parentId", j.children
         FROM   t
         LEFT   JOIN j USING (id)
         WHERE  t.lvl = 1
         ORDER  BY type, id
         ) sub
      GROUP  BY 1
      ) sub
   );
END
$func$;

调用（精确返回所需结果）：

SELECT jsonb_pretty(f_build_jsonb_tree());

数据库

我选择了键名children，而不是children，因为可以嵌套多个。

jsonb_pretty（）美化显示是可选的。

这是假设参照完整性；应使用FK约束实现。

对于您的特定情况，该解决方案可能更简单，如果它显示（未公开）有用的属性，那么可以利用code列。我们可以在没有rCTE的情况下导出嵌套级别，并添加临时表t。但我的目标是只基于ID引用的通用解决方案。

函数中有很多内容。我添加了内联注释。基本上，它是这样做的：

1. 使用添加的嵌套级别（lvl）创建临时表

该函数将类型作为参数，仅返回给定的类型。否则，将处理整个表。

旁白：尽可能避免在Postgres中使用混合大小写标识符，如“parentId”。请参阅：

• PostgreSQL列名区分大小写吗

使用递归函数的相关后续答案：

• 如何将一组扁平的树变成一棵多叶的树？