How to compute a JSON field from other fields

Given a table containing the following fields

How to create a SQL query that will generate a JSON object that will look like this?

{"A": 10, "B": -5}

>Solution :

A quick and dirty example:

create table json_object (id integer, sign1 varchar,  value1 varchar, qty1 integer, sign2 varchar,  value2 varchar,  qty2 integer);

insert into json_object values (1, 'positive', 'A', 10, 'negative', 'B', 5);

select json_build_object(value1, qty1 * (case when sign1 = 'positive' then 1 else -1 end), value2, qty2 * (case when sign2 = 'positive' then 1 else -1 end)) from json_object;

 json_build_object   
----------------------
 {"A" : 10, "B" : -5}