postgresql json: keys as values - postgresql

Question

I just discovered the json capabilities of postgresql but have trouble understanding how to generate json with queries. I hope the question I am asking makes sense and please excuse me if I am missing something obvious.
my problem ? how to generate json with some values being keys to others.
here an example
drop table if exists my_table;
create table my_table(id int, sale_year int, sale_qty int);
insert into my_table values (10, 2007, 2);
insert into my_table values (10, 2008, 1);
insert into my_table values (10, 2009, 0);
insert into my_table values (20, 2009, 2);
insert into my_table values (30, 2011, 1);
insert into my_table values (30, 2012, 3);
The following statement
SELECT id, json_agg(to_json(my_table)) FROM public.my_table group by id;
gives me a json per id (e.g. for id = 20)
20, [{"id":20, "sale_year": 2009, "sale_qty": 2}]
my question is:
is it possible to return a json with the following structure ?
{"2009": 2}

Answer 1

I think you want something like this:
select id, json_agg(json_build_object(sale_year, sale_qty))
from my_table
group by id
order by id;
This returns:
id | json_agg
---+-------------------------------------------
10 | [{"2007" : 2}, {"2008" : 1}, {"2009" : 0}]
20 | [{"2009" : 2}]
30 | [{"2011" : 1}, {"2012" : 3}]

Answer 2

I hope that this will help someone else
in some cases, one would want to get, not an array of jsonb data but a single jsonb element.
inspired from this post this post this is an example of how to do it
with tx1
as
(
select
*
from
(values
(10, 2007, 2),
(10, 2008, 1),
(10, 2009, 0),
(20, 2009, 2),
(30, 2011, 1),
(30, 2012, 3))
as t (id, sale_year, sale_qty)),
tx2
as
(select id,
jsonb_agg(json_build_object(sale_year, sale_qty)) as x_data
from tx1
group by id
order by id)
SELECT
id,
x_data,
jo.obj
FROM tx2
CROSS JOIN
LATERAL
(
SELECT JSON_OBJECT_AGG(jt.key, jt.value) obj
FROM JSONB_ARRAY_ELEMENTS(x_data) je
CROSS JOIN
LATERAL JSONB_EACH(je.value) jt
) jo
This gives
{ "2007" : 2, "2008" : 1, "2009" : 0 }
{ "2011" : 1, "2012" : 3 }