Mysql贝叶斯和按星级评分

Question

说我有两张桌子。 businesses和reviews为企业。

businesses表：

+----+-------+ 
| id | title | 
+----+-------+ 

reviews表：

+----+-------------+---------+------+ 
| id | business_id | message | rate | 
+----+-------------+---------+------+ 

每个评论有一个rate（1到5星）
我想他们的评论率对企业进行排序，根据Bayesian Ranking条件至少有2条评论。

这里是我的查询：

SELECT b.id, 
(SELECT COUNT(r.rate) as rr FROM reviews r WHERE r.business_id = b.id) as rr, 
(SELECT 
     ((COUNT(r.rate)/(COUNT(r.rate) + 2)) AVG(r.rate) + 
     (2 /(COUNT(r.rate) + 2)) 4) 
    FROM reviews r where r.business_id = b.id AND rr > 2 
) as score 
FROM businesses b 
order by score desc 
LIMIT 4 

这将输出我：

+------+----+------------+ 
| id | rr | score  | 
+------+----+------------+ 
| 992 | 14 | 4.31250000 | 
+------+----+------------+ 
| 237 | 3 | 4.2000000 | 
+------+----+------------+ 
| 19 | 5 | 4.0000000 | 
+------+----+------------+ 
| 1009 | 12 | 3.9285142 | 
+------+----+------------+ 

我有两个问题：

1. 当你看到在((COUNT(r.rate)/(COUNT(r.rate) + 2)) AVG(r.rate) + (2 /(COUNT(r.rate) + 2)) 4) FROM reviews r where r.business_id = b.id AND rr > 2)一些功能正在运行更多比一次，如COUNT或AVG。他们是否在后台运行一次，也许缓存resuslt？或运行每一个电话？

2. 是否有任何等效查询，但更优化？

在此先感谢。

Answer 1

我希望MySQL能够优化多重计数，但不能确定。

但是，您可以重新安排您查询加入反对子查询。这样你不会为每一行执行2个子查询。

SELECT b.id, 
     sub0.rr, 
     sub0.score 
FROM businesses b 
INNER JOIN 
(
    SELECT r.business_id, 
      COUNT(r.rate) AS rr , 
      ((COUNT(r.rate)/(COUNT(r.rate) + 2)) AVG(r.rate) + (2 /(COUNT(r.rate) + 2)) 4) AS score 
    FROM reviews r 
    GROUP BY r.business_id 
    HAVING rr > 2 
) sub0 
ON sub0.business_id = b.id 
ORDER BY score DESC 
LIMIT 4 

注意，这里的结果是非常略有不同，因为它会排除只有2条评论记录，而您的查询仍然会返回，但他们的得分为NULL。我已经离开了明显缺少的运营商（即AVG（r.rate）之前和之前4）您的原始查询AS评分。

使用上面的想法，您可以重新编码它以返回子查询中的计数和平均速率，并仅使用那些返回的列的值来计算。

SELECT b.id, 
     sub0.rr, 
     ((rr/(rr + 2)) arr + (2 /(rr + 2)) 4) AS score 
FROM businesses b 
INNER JOIN 
(
    SELECT r.business_id, 
      COUNT(r.rate) AS rr , 
      AVG(r.rate) AS arr 
    FROM reviews r 
    GROUP BY r.business_id 
    HAVING rr > 2 
) sub0 
ON sub0.business_id = b.id 
ORDER BY score DESC 
LIMIT 4