602. 好友申请 II ：谁有最多的好友

写一个查询语句，求出谁拥有最多的好友和他拥有的好友数目。
就是求出两个字段中最多的那个id
这里用到了union all

SELECT id, COUNT(*) AS num 
FROM 
(
(    
SELECT requester_id AS id 
FROM request_accepted
)
UNION ALL 
(
SELECT accepter_id AS id 
FROM request_accepted
)
) AS tmp
GROUP BY id 
ORDER BY num desc
LIMIT 1 

这样比较好理解，都命名为id，然后union all