json - 在 Postgres 中将邻接列表作为嵌套 JSON 返回

Question

给定两列 Postgres 表 (id, parent_id) 的最简单基本情况，是否有一种方法可以查询 id 并将所有子项作为嵌套的 json 结构返回，如下所示？

{
    "id": 1,
    "children": [{
        "id": 2,
        "children": [{
            "id": 3,
            "children": []
        }]
    }]
}

我了解如何通过表进行递归，但我无法拼凑出如何使用任何 psql json 函数来返回如上所示的结果。也许我应该只使用我选择的语言在后处理中进行转换？

SQLFiddle目前的进展。

Answer 1

太难了，我花了好几个小时才解决:-P

CREATE TABLE foo (
id INT PRIMARY KEY,
parent_id INT);

insert into foo values (1, null);
insert into foo values (2, 1);
insert into foo values (3, 2);

WITH RECURSIVE 
    tree AS (
    SELECT 1 AS round, id, parent_id, ARRAY(SELECT id FROM foo WHERE parent_id = f.id) AS children
    FROM foo f
    WHERE id = 1
    UNION ALL
    SELECT round+1, f.id, f.parent_id, ARRAY(SELECT id FROM foo WHERE parent_id = f.id) AS children
    FROM tree t
    JOIN foo f ON (f.id = ANY(t.children))
    ),
    rev AS (
    SELECT r.round-1 AS round,
            to_jsonb(ARRAY(
                    SELECT a
                    FROM (
                        SELECT f.parent_id AS id, json_agg(jsonb_build_object('id', f.id, 'children', '{}'::text[])) AS children
                        FROM tree t
                        JOIN foo f ON (f.id = t.id)
                        WHERE t.round = r.round
                        GROUP BY f.parent_id
                        ) a
                    )) AS list
    FROM (SELECT MAX(round)::int AS round FROM tree) r
    UNION ALL
    SELECT r.round-1,
            to_jsonb(ARRAY(
                    SELECT a
                    FROM (
                        SELECT f.parent_id AS id, json_agg(jsonb_build_object('id', f.id, 'children', t->'children')) AS children
                        FROM jsonb_array_elements(list) t
                        JOIN foo f ON (f.id = (t->>'id')::int)
                        GROUP BY f.parent_id
                        ) a
                    )) AS list
    FROM rev r
    WHERE round > 1
    )
SELECT list as nested_json_tree
FROM rev
WHERE round = 1

复杂性在于需要先构建树(自上而下)，然后从树自下而上构建对象。由于递归查询的限制，递归自下而上是棘手的，例如 UNION ALL 部分中的递归别名不能被分组，也不能包含在子查询中。我通过反向循环展开来解决这个问题。

这个查询应该正确地构建复杂的树，每个节点有多个子节点，以及任意数量的嵌套级别。