Return as array of JSON objects in SQL (Postgres)

问题

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?

回答1:

json_build_object() in Postgres 9.4 or newer

SELECT value_two, json_agg(json_build_object('value_three', value_three
                                           , 'value_four' , value_four)) 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((value_three, value_four)::foo)) 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((SELECT t FROM (SELECT value_three, value_four) t))) 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 SQL fiddle.

来源：https://stackoverflow.com/questions/26486784/return-as-array-of-json-objects-in-sql-postgres

标签

sql

arrays

json

postgresql

aggregate-functions