假设淘宝有一张商品评论表,存储在MySQL中.表设计如下:

ID为productlist_comment主键,

UserID为用户的ID,

ProductID为商品ID

Content为评论的内容,

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:?