MYSQL查询数据中通讯最多的两个人

SELECT GREATEST( sender_id, recipient_id ) , LEAST( sender_id, recipient_id ) , COUNT( * ) AS a
FROM `pmsg`
GROUP BY CONCAT( GREATEST( sender_id, recipient_id ) , LEAST( sender_id, recipient_id ) )
ORDER BY a DESC
LIMIT 3

注：sender_id：是指发件人id，recipient_id是指收件人id