1322.(Easy)广告效果

表: Ads

+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
| ad_id         | int     |
| user_id       | int     |
| action        | enum    |
+---------------+---------+
(ad_id, user_id) 是该表的主键
该表的每一行包含一条广告的 ID(ad_id)，用户的 ID(user_id) 和用户对广告采取的行为 (action)
action 列是一个枚举类型 ('Clicked', 'Viewed', 'Ignored') 。

一家公司正在运营这些广告并想计算每条广告的效果。

广告效果用点击通过率（Click-Through Rate：CTR）来衡量，公式如下:

$$C T R=\left\{\begin{array}{ll}0, & \text { if Ad total clicks }+\text { Ad total views }=0 \\ \frac{\text { Ad total clicks }}{\text { Ad total clicks }+\text { Ad total views }} \times 100, & \text { otherwise }\end{array}\right.$$

写一条SQL语句来查询每一条广告的 ctr ，

ctr 要保留两位小数。结果需要按 ctr 降序、按 ad_id 升序 进行排序。

查询结果示例如下：

Ads 表:
+-------+---------+---------+
| ad_id | user_id | action  |
+-------+---------+---------+
| 1     | 1       | Clicked |
| 2     | 2       | Clicked |
| 3     | 3       | Viewed  |
| 5     | 5       | Ignored |
| 1     | 7       | Ignored |
| 2     | 7       | Viewed  |
| 3     | 5       | Clicked |
| 1     | 4       | Viewed  |
| 2     | 11      | Viewed  |
| 1     | 2       | Clicked |
+-------+---------+---------+
结果表:
+-------+-------+
| ad_id | ctr   |
+-------+-------+
| 1     | 66.67 |
| 3     | 50.00 |
| 2     | 33.33 |
| 5     | 0.00  |
+-------+-------+
对于 ad_id = 1, ctr = (2/(2+1)) * 100 = 66.67
对于 ad_id = 2, ctr = (1/(1+2)) * 100 = 33.33
对于 ad_id = 3, ctr = (1/(1+1)) * 100 = 50.00
对于 ad_id = 5, ctr = 0.00, 注意 ad_id = 5 没有被点击 (Clicked) 或查看 (Viewed) 过
注意我们不关心 action 为 Ingnored 的广告
结果按 ctr（降序），ad_id（升序）排序

来源：力扣（LeetCode）

链接：https://leetcode-cn.com/problems/ads-performance

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Solution

select ad_id, 
    round(ifnull(100*sum(action='Clicked')/(sum(action='Clicked')+sum(action='Viewed')),0),2) ctr
from Ads
group by ad_id
order by ctr desc, ad_id