使用自连接在 row_to_json() 中复制 JSON 元素

Question

这是这个优秀问答的后续：13227142。

我几乎不得不做同样的事情（受 PostgreSQL 9.2 的限制）但我只使用了一个 table。因此，查询使用自连接（为了生成正确的 JSON 格式），这会导致重复的 id 字段。我怎样才能避免这种情况？

示例：

CREATE TABLE books
(
  id serial primary key,
  isbn text,
  author text,
  title text,
  edition text,
  teaser text
);

SELECT row_to_json(row)
FROM
(
  SELECT id AS bookid,
  author,
  cover
  FROM books
  INNER JOIN 
  (
    SELECT id, title, edition, teaser
    FROM books
  ) cover(id, title, edition, teaser)
  USING (id)
) row;

结果：

{
  "bookid": 1,
  "author": "Bjarne Stroustrup",
  "cover": {
    "id": 1,
    "title": "Design and Evolution of C++",
    "edition": "1st edition",
    "teaser": "This book focuses on the principles, processes and decisions made during the development of the C++ programming language"
  }
}

我想删除 "cover" 中的 "id"。

Answer 1

你需要id才能加入，所以没有id你不能做这么短的查询。你需要构造它。像：

select row_to_json(row,true)
FROM
(
with a as (select id,isbn,author,row_to_json((title,edition,teaser)) r from books
)
select a.id AS bookid,a.author, concat('{"title":',r->'f1',',"edition":',r->'f2',',"teaser":',r->'f3','}')::json as cover
  from a
) row;
                      row_to_json
--------------------------------------------------------
 {"bookid":1,                                          +
  "author":"\"b\"",                                    +
  "cover":{"title":"c","edition":"d","teaser":"\"b\""}}
(1 row)

同样，如果没有加入，您使用的资源会减少两倍

Answer 2

事实证明这是一项棘手的任务。据我所知，用一个简单的查询是不可能实现的。一种解决方案是使用预定义的数据类型：

CREATE TYPE bookcovertype AS (title text, edition text, teaser text);

SELECT row_to_json(row)
FROM
(
  SELECT books.id AS bookid, books.author, 
    row_to_json(row(books.title, books.edition, books.teaser)::bookcovertype) as cover
  FROM books
) row;

Answer 3

为了完整起见，我自己偶然发现了另一个答案：可以通过字符串函数消除附加字段。但是，我更喜欢 AlexM 的回答，因为它会更快并且仍然与 PostgreSQL 9.2 兼容。

SELECT regexp_replace(
  (
      SELECT row_to_json(row)
      FROM
      (
        SELECT id AS bookid,
        author,
        cover
        FROM books
        INNER JOIN 
        (
          SELECT id, title, edition, teaser
          FROM books
        ) cover(id, title, edition, teaser)
        USING (id)
      ) row
  )::text,
  '"id":\d+,',
  '')

使用自连接在 row_to_json() 中复制 JSON 元素

Duplicate JSON element in row_to_json() with self-join

sql

postgresql

postgresql-9.2

sjson