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题目链接：

题目大意：略。

解题思路：1/0, 0/0 结果都为 NULL。

AC 代码

-- 解决方案(1)WITH t1 AS(SELECT 1 id, COUNT(DISTINCT requester_id, accepter_id) up FROM RequestAccepted),t2 AS(SELECT 1 id, COUNT(DISTINCT sender_id, send_to_id) down FROM FriendRequest)SELECT IF(down = 0, 0.00, ROUND(up/down, 2)) accept_rateFROM t1, t2WHERE t1.id = t2.id-- 解决方案(2)selectround(    ifnull(    (select count(*) from (select distinct requester_id, accepter_id from request_accepted) as A)    /    (select count(*) from (select distinct sender_id, send_to_id from friend_request) as B),    0), 2) as accept_rate;

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