PostgreSQL由用作表达式的子查询返回的多行

string_agg() json_agg()

SELECT name,
       (SELECT string_agg(car_name) FROM cars WHERE user = id)
FROM users
WHERE user_id = 1
ORDER BY name;

JOIN

SELECT u.name, string_agg(c.car_name)
FROM users u LEFT OUTER JOIN
     cars c
     ON c.id = u.user
WHERE u.user_id = 1
GROUP BY u.name
ORDER BY u.name;