Nested json, PostgreSQL

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drop table if exists wheel;
drop table if exists car;
drop table if exists person;

create table person
(
  id integer not null,
  name character varying(30),
  constraint person_pk primary key (id)
);

create table car
(
  id integer not null,
  type character varying(30),
  personid integer,
  constraint car_pk primary key (id)
);

create table wheel
(
  id integer not null,
  whichone character varying(30),
  serialnumber integer,
  carid integer,
  constraint "wheel_pk" primary key (id)
);

insert into person(id, name) values 
    (1, 'Johny'), 
    (2, 'Freddy'),
    (3, 'Billy');

insert into car(id, type, personid) values 
    (1, 'Toyota', 1),
    (2, 'Fiat', 2),
    (3, 'Opel', 3);

insert into wheel(id, whichone, serialnumber, carid) values 
    (1, 'front', '11', 1),
    (2, 'back', '12', 1),
    (3, 'front', '21', 2),
    (4, 'back', '22', 2),
    (5, 'front', '3', 3);

select
		json_build_object(
			'persons', json_agg(
				json_build_object(
					'person_name', p.name,
					'cars', cars
				)
			)
		) persons
	from person p
	left join (
		select 
			personid,
--			json_agg(
				json_build_object(
					'carid', c.id,    
					'type', c.type,
					'comment', 'nice car', -- this is constant
					'wheels', wheels
					) cars --+
--				) cars
		from
			car c
			left join (
				select 
					carid, 
					json_agg(
						json_build_object(
							'which', w.whichone,
							'serial number', w.serialnumber
						)
					) wheels
				from wheel w
				group by 1
			) w on c.id = w.carid
--		group by personid
	) c on p.id = c.personid;


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