MySQL仿淘宝页面评论评分统计SQL

scaoping

　　假设淘宝有一张商品评论表,存储在MySQL中.表设计如下:
　　ID为[size=1em]productlist_comment主键,
　　[size=1em]UserID为用户的ID,
　　[size=1em]ProductID为商品ID
　　[size=1em]Content为评论的内容,
　　[size=1em]CreateTime为评论的时间.
　　Star为商品评分分数,取值为1-5;
　　ProductID和UserID已经分别建立索引. 

SELECT ID, ProductID, UserID, Star, Content, Createtime
FROM mysqldb.productlist_comment limit 10
　　查询得到如下评论数据:

要求在商品页面需要计算出,
　　好评数(Star>3),差评数(Star<3),中评数(Star=3),
　　以及评论总数,平均Star分数(如4.7), 好评百分比.
　　正对上面需求, 整理SQL如下: 

select SUM(s.Star45) as Star45Num,
SUM(s.Star3)  as Star3Num,
SUM(s.Star12) as Star12Num,
SUM(s.Record) as TotalNum,
AVG(s.starValue) AS AverageStar,
SUM(s.Star45)/SUM(s.Record) AS Star45Percent
from(
select case when pc.star>3 then 1 else 0 end as Star45,
case when pc.star=3 then 1 else 0 end as Star3,
case when pc.star<3 then 1 else 0 end as Star12,
pc.star as starValue,
1 AS Record
from mysqldb.ProductList_comment pc
where pc.ProductID=15792
) as s
　　得出的数据展现如下: