Duplicate JSON element in row_to_json() with self-join - sql

Question

This is a follow-up to this excellent Q&A: 13227142.
I almost have to do the same thing (with the constraint of PostgreSQL 9.2) but I'm using only one table. Therefore the query uses a self-join (in order to produce the correct JSON format) which results in a duplicate id field. How can I avoid this?
Example:
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;
Result:
{
"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"
}
}
I want to get rid of "id" in "cover".

Answer 1

This turned out to be a tricky task. As far as I can see it's impossible to achieve with a simple query.
One solution is to use a predefined data type:
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 2

you need id to join, so without id you can't make such short query. You need to struct it. Smth like:
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)
Also without join you use twice as less resources

Answer 3

For the sake of completeness I've stumbled upon another answer myself: The additional fields can be eliminated by string functions. However, I prefer AlexM's anwer because it will be faster and is still compatible with PostgreSQL 9.2.
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+,',
'')