Exzyggwp

Ниже вариант запроса (PostgreSQL) который собирает всё в один JSON:

SELECT row_to_json(client.*)
FROM (
  SELECT 
	clients.*,
	json_agg(
	  row_to_json(trainers.*)
	) trainers
  FROM 
	clients 
  JOIN trainers ON trainers.t_id < clients.t_id
  where 
	clients.t_id = 10
  GROUP BY clients.t_id, clients.t_name, clients.t_phone
) client;

SQL fiddle

Вариант PL\SQL:

SELECT json_object(
      't_id' VALUE client.t_id,
      't_name' VALUE client.t_name,
      't_phone' VALUE client.t_phone,
      't_trainers' VALUE client.trainers
  ) FROM (
  SELECT 
  clients.*,
  json_arrayagg(json_object(
      't_id' VALUE trainers.t_id,
      't_name' VALUE trainers.t_name,
      't_phone' VALUE trainers.t_phone
  )) trainers
  FROM 
	clients 
  JOIN trainers ON trainers.t_id < clients.t_id
  where 
	clients.t_id = 10
  GROUP BY clients.t_id, clients.t_name, clients.t_phone
  ) client;