Postgres 14

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select version();

version

PostgreSQL 14.10 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 8.5.0 20210514 (Red Hat 8.5.0-20), 64-bit

SELECT 1

CREATE TABLE actors( id INT, name VARCHAR);
INSERT INTO actors VALUES(1,'Bob'),(2,'John');

CREATE TABLE movies( id INT, title VARCHAR);
INSERT INTO movies VALUES(1,'aaa'),(2,'bbb');

CREATE TABLE movies_actors( movie_id INT, actor_id INT);
INSERT INTO movies_actors VALUES(1,1),(1,2),(2,1);

CREATE TABLE

INSERT 0 2

CREATE TABLE

INSERT 0 2

CREATE TABLE

INSERT 0 3

SELECT JSON_AGG(result)
  FROM
(
  SELECT JSON_BUILD_OBJECT(
                         'id', m.id, 
                         'title', m.title,
                         'actors',JSON_AGG(JSON_BUILD_OBJECT('id', a.id, 'name', a.name))
                         ) AS result
  FROM movies AS m
  LEFT JOIN movies_actors AS ma
    ON m.id = ma.movie_id
  LEFT JOIN actors AS a
    ON ma.actor_id = a.id
GROUP BY m.id, m.title
) AS j

json_agg

[{"id" : 1, "title" : "aaa", "actors" : [{"id" : 1, "name" : "Bob"}, {"id" : 2, "name" : "John"}]}, {"id" : 2, "title" : "bbb", "actors" : [{"id" : 1, "name" : "Bob"}]}]

SELECT 1