@txoooy 的方法查询要 0.07 秒，采用下面方法只需 0.02 秒

SELECT T1.`uid`,T1.`headimg`, T1.`nickname` ,T1.`referer` ,T2.`num`
FROM `user_center` AS T1 LEFT JOIN (
SELECT C.`referer` AS uid, count(`referer`) AS num
FROM `user_center` C, `user_open` O
WHERE C.`uid`= O.`uid`
AND O.`subscribe`=1
GROUP BY C.`referer`
) AS T2
ON T1.`uid`= T2.`uid`
WHERE T2.`num` IS NOT NULL
ORDER BY T2.`num` DESC