首先左连接交换到user并使用聚合来获取每个用户的匹配次数。然后再次聚合以获取每个国家/地区的平均匹配次数。

SELECT x.country
       FROM (SELECT u.uid,
                    u.country,
                    count(e.eid) matches
                    FROM user u
                         LEFT JOIN exchange e
                                   ON e.match_id = u.match_id
                    GROUP BY u.uid,
                             u.country) x
       GROUP BY x.country
       HAVING avg(x.matches) < 2;