LeetCode MySQL 1126. 查询活跃业务

+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
| business_id   | int     |
| event_type    | varchar |
| occurences    | int     | 
+---------------+---------+
此表的主键是 (business_id, event_type)。
表中的每一行记录了某种类型的事件在某些业务中多次发生的信息。

Events table:
+-------------+------------+------------+
| business_id | event_type | occurences |
+-------------+------------+------------+
| 1           | reviews    | 7          |
| 3           | reviews    | 3          |
| 1           | ads        | 11         |
| 2           | ads        | 7          |
| 3           | ads        | 6          |
| 1           | page views | 3          |
| 2           | page views | 12         |
+-------------+------------+------------+

结果表
+-------------+
| business_id |
+-------------+
| 1           |
+-------------+ 
'reviews'、 'ads' 和 'page views' 的总平均发生次数
分别是 (7+3)/2=5, (11+7+6)/3=8, (3+12)/2=7.5。
id 为 1 的业务有 7 个 'reviews' 事件（大于 5）
和 11 个 'ads' 事件（大于 8），所以它是活跃业务。

# Write your MySQL query statement below
select e.business_id
from Events e left join 
(
    select event_type, avg(occurences) avgocc
    from Events
    group by event_type
) t
on e.event_type = t.event_type
group by e.business_id
having sum(e.occurences > t.avgocc) >= 2