不需要Join，直接进行单表查询+子查询过滤
异常的邮件概率
http://www.nowcoder.com/practice/d6dd656483b545159d3aa89b4c26004e
- 首先找出黑名单的user:
select id
from user
where is_blacklist = 12.然后查询email表：
select
date,
round((sum(type='no_completed') / COUNT(*)),3) as p
from email
where send_id and receive_id not in (
select id
from user
where is_blacklist = 1
)
GROUP BY date
order by date asc;其中round函数是保留几位精度；
count函数中只能传进去列，不能传条件进去；
sum函数对满足条件的进行求和，sum可以传进去if判断，也可以直接传进去条件，满足的+1；
