PostgreSQL Query output to JSON

Question

I would like to get the output of a particular Query from the postgresql server having joins of a multiple Tables with one to many relations in the json format to a particular file.

The Query output should be an actual json tree like below and not a flat join of master + child :-

Master Record 1 (master field 1, master field 2)
              { array
                             Child Record 1 {child field 1, child field 2, ….)
                             Child Record 2 { child field 1, child field 2, ….)
               }
Master Record 2 (....)
                Child Record 1
                Child Record 2

Please let me know how to achieve this in the Postgresql 11.

SQL Script

CREATE TABLE public.book
(
    pk_book_id integer NOT NULL,
    name character varying(255) COLLATE pg_catalog."default",
    isbn character varying(255) COLLATE pg_catalog."default",
    CONSTRAINT book_pkey PRIMARY KEY (pk_book_id)
);

CREATE TABLE public.author
(
    pk_author_id integer NOT NULL,
    fullname character varying(255) COLLATE pg_catalog."default",
    mobileno character(10) COLLATE pg_catalog."default",
    fk_book_id integer,
    CONSTRAINT author_pkey PRIMARY KEY (pk_author_id),
    CONSTRAINT author_fk_book_id_fkey FOREIGN KEY (fk_book_id)
        REFERENCES public.book (pk_book_id) MATCH SIMPLE
        ON UPDATE NO ACTION
        ON DELETE CASCADE
);

INSERT INTO public.author(
    pk_author_id, fullname, mobileno, fk_book_id),
    (100, 'John Matthew', '98927828', 1),
    (101, 'Mark Knight', '99875528', 1),
    (103, 'Lara Croft', '99872628', 2),
    (104, 'Brad Show', '567982', 2)
    
INSERT INTO public.book(
    pk_book_id, name, isbn)
    VALUES (1, 'Hot Deals', 'ISBN89644'),
    (2, 'Summer Tales', 'ISBN405987');
    

Answer 1

It's a bit hard to follow your sample, but something along the lines should do it:

select to_jsonb(main)||ch.data
from main_table main 
  join (
     select main_id, jsonb_agg(to_jsonb(child)) as data
     from child_table child
     group by main_id
  ) ch on ch.main_id = main.id

If you want everything as one gigantic JSON array, use select jsonb_agg(to_jsonb(main)||ch.data) in the outer select.