考虑到MySQL中的评级列表中没有滚动NOR的情况下，平均评级为零。

考虑以下模式：

Student (RollNo int Not Null, Name varchar(20) Not Null, YearOfAdmission int Not Null,
PRIMARY KEY(RollNo))

现答覆如下：

列出所有电影的平均评分（包括多个评分实例）的学生 不同日期的电影）低于他/她的朋友对这些电影的平均评分 （包括在不同日期对电影进行分级的多个实例）。（输出格式：RollNo1， AverageRating1、RollNo2、AverageRating2）

一个可能的答案是-

Select x.OwnRoll as RollNo1, l1.average as AverageRating1,
x.FriendRoll as RollNo2, l2.average as AverageRating2
from 
(
Select * from Friend 
union 
(
Select f.FriendRoll, f.OwnRoll from Friend as f
)
order by OwnRoll
) as x,

(
Select r.Rollno, avg(r.Rating) as average
from Rating as r 
group by r.Rollno
) as l1,

(
Select r.Rollno, avg(r.Rating) as average
from Rating as r 
group by r.Rollno
) as l2
where l1.Rollno = x.OwnRoll and l2.Rollno = x.FriendRoll
and l1.average > l2.average ;

但是这个版本没有考虑从未看过电影的朋友，所以他们的平均评分是0＜/p>。

---

提前感谢您对问题及其答案所做的任何更新。

如果没有卷号，请执行此小更新以将评分平均值捕获为0

Select distinct r.Rollno, avg(r.Rating) as average
from Rating as r 
group by r.Rollno
union
(
Select RollNo, 0 as average
from Student 
where Rollno not in (Select RollNo from Rating)
)

您甚至可以进一步了解Coalesce

类似的问题也被贴出来了

最终答案是

Select x.OwnRoll as RollNo1, l1.average as AverageRating1,
x.FriendRoll as RollNo2, l2.average as AverageRating2
from 
(
Select * from Friend 
union (Select f.FriendRoll, f.OwnRoll from Friend as f)
order by OwnRoll
) as x,

(
Select distinct r.Rollno, avg(r.Rating) as average
from Rating as r 
group by r.Rollno
union
(
Select RollNo, 0 as average
from Student 
where Rollno not in 
(Select RollNo from Rating))
) as l1,

(
Select distinct r.Rollno, avg(r.Rating) as average
from Rating as r 
group by r.Rollno
union
(
Select RollNo, 0 as average
from Student 
where Rollno not in 
(Select RollNo from Rating))
) as l2

where l1.Rollno = x.OwnRoll and l2.Rollno = x.FriendRoll
and l1.average > l2.average 

试着一步一步地理解它！！否则你会迷失方向：）

添加此问题的动机是为了得到一个比我刚刚发布的更好、更简单的解决方案

Select x.OwnRoll as RollNo1, l1.average as AverageRating1,
x.FriendRoll as RollNo2, l2.average as AverageRating2
from 
(
Select * from Friend 
union 
(
Select f.FriendRoll, f.OwnRoll from Friend as f
)
order by OwnRoll
) as x,

(
Select r.Rollno, avg(r.Rating) as average
from Rating as r 
group by r.Rollno
) as l1,

(
Select r.Rollno, avg(r.Rating) as average
from Rating as r 
group by r.Rollno
) as l2
where l1.Rollno = x.OwnRoll and l2.Rollno = x.FriendRoll
and l1.average > l2.average ;

Select distinct r.Rollno, avg(r.Rating) as average
from Rating as r 
group by r.Rollno
union
(
Select RollNo, 0 as average
from Student 
where Rollno not in (Select RollNo from Rating)
)

Select x.OwnRoll as RollNo1, l1.average as AverageRating1,
x.FriendRoll as RollNo2, l2.average as AverageRating2
from 
(
Select * from Friend 
union (Select f.FriendRoll, f.OwnRoll from Friend as f)
order by OwnRoll
) as x,

(
Select distinct r.Rollno, avg(r.Rating) as average
from Rating as r 
group by r.Rollno
union
(
Select RollNo, 0 as average
from Student 
where Rollno not in 
(Select RollNo from Rating))
) as l1,

(
Select distinct r.Rollno, avg(r.Rating) as average
from Rating as r 
group by r.Rollno
union
(
Select RollNo, 0 as average
from Student 
where Rollno not in 
(Select RollNo from Rating))
) as l2

where l1.Rollno = x.OwnRoll and l2.Rollno = x.FriendRoll
and l1.average > l2.average