Return multiple columns of the same row as JSON array of objects

Question

I have the following table MyTable :

 id │ value_two │ value_three │ value_four 
────┼───────────┼─────────────┼────────────
  1 │ a         │ A           │ AA
  2 │ a         │ A2          │ AA2
  3 │ b         │ A3          │ AA3
  4 │ a         │ A4          │ AA4
  5 │ b         │ A5          │ AA5

I want to query an array of objects { value_three, value_four } grouped by value_two . value_two should be present on its own in the result. The result should look like this:

 value_two │                                                                                    value_four                                                                                 
───────────┼───────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
 a         │ [{"value_three":"A","value_four":"AA"}, {"value_three":"A2","value_four":"AA2"}, {"value_three":"A4","value_four":"AA4"}]
 b         │ [{"value_three":"A3","value_four":"AA3"}, {"value_three":"A5","value_four":"AA5"}]

It does not matter whether it uses json_agg() or array_agg() .

However the best I can do is:

with MyCTE as ( select value_two, value_three, value_four from MyTable ) 
select value_two, json_agg(row_to_json(MyCTE)) value_four 
from MyCTE 
group by value_two;

Which returns:

 value_two │                                                                                    value_four                                                                                 
───────────┼───────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
 a         │ [{"value_two":"a","value_three":"A","value_four":"AA"}, {"value_two":"a","value_three":"A2","value_four":"AA2"}, {"value_two":"a","value_three":"A4","value_four":"AA4"}]
 b         │ [{"value_two":"b","value_three":"A3","value_four":"AA3"}, {"value_two":"b","value_three":"A5","value_four":"AA5"}]

With an extra value_two key in the objects, which I would like to get rid of. Which SQL (Postgres) query should I use?

Answer 1

json_build_object() in Postgres 9.4 or newer

Or jsonb_build_object() to return jsonb .

SELECT value_two, json_agg() AS value_four
FROM   mytable 
GROUP  BY value_two;

The manual:

Builds a JSON object out of a variadic argument list. By convention, the argument list consists of alternating keys and values.

For any version (incl. Postgres 9.3)

row_to_json() with a ROW expression would do the trick:

SELECT value_two
     , json_agg(row_to_json((value_three, value_four))) AS value_four
FROM   mytable
GROUP  BY value_two;

But you lose original column names. A cast to a registered row type avoids that. (The row type of a temporary table serves for ad hoc queries, too.)

CREATE TYPE foo AS (value_three text, value_four text);  -- once in the same session

SELECT value_two
     , json_agg(row_to_json()) AS value_four
FROM   mytable
GROUP  BY value_two;

Or use a subselect instead of the ROW expression. More verbose, but without type cast:

SELECT value_two
     , json_agg(row_to_json()) AS value_four
FROM   mytable
GROUP  BY value_two;

More explanation in Craig's related answer:

• PostgreSQL 9.2 row_to_json() with nested joins

db<>fiddle here
_{Old sqlfiddle}

Answer 2

to_json with array_agg with composite type

begin;
create table  mytable(
id bigint, value_two text, value_three text, value_four  text);
insert into mytable(id,value_two, value_three,value_four)
values
 ( 1, 'a',       'A',           'AA'),
  (2, 'a'    ,     'A2'  ,       'AA2'),
  (3, 'b'  ,       'A3',         'AA3'),
 ( 4, 'a'   ,      'A4',          'AA4'),
  (5, 'b' ,        'A5',          'AA5');
commit;
create type mytable_type as (value_three text, value_four text);

---

select value_two,
       to_json( array_agg(row(value_three,value_four)::mytable_type))
from mytable
group by 1;