SQLite 3.39

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CREATE TABLE events (`id` INTEGER, `content` TEXT);
✓
INSERT INTO events (`id`, `content`) VALUES
  ('1', 'abc'), ('2', 'xyz');
✓
CREATE TABLE timeslots (`id` INTEGER, `start` TEXT, `end` TEXT);
✓
INSERT INTO timeslots (`id`, `start`, `end`) VALUES
  ('1', '05-2022', '06-2022'), ('1', '10-2022', '12-2022'), ('2', '11-2022', '12-2022');
✓
SELECT json_object(
         'id', e.id,
         'content', e.content,
         'timeslots', json_group_array(
                        json_object(
                          'start', t.start,
                          'end', t.end
                        )
                      )
       ) col
FROM events e INNER JOIN timeslots t
ON t.id = e.id
-- WHERE e.id = ?
GROUP BY e.id;


col


{"id":1,"content":"abc","timeslots":[{"start":"05-2022","end":"06-2022"},{"start":"10-2022","end":"12-2022"}]}

{"id":2,"content":"xyz","timeslots":[{"start":"11-2022","end":"12-2022"}]}