SQL Server的平均查询

Question

有人可以帮我查询这个的平均值吗？

我有这张桌子。

teacher | class | students | rating 
----------------------------------- 
    T1  1  001  6 
    T1  1  002  6 
    T1  2  003  1 
    T1  3  004  1 
    T2  1  001  6 
    T2  1  002  6 
    T2  2  003  1 
    T2  3  004  1 

如果我这样做Select teacher, class, avg(rating) from table group by teacher, class然后我得到这样的：

teacher | class | rating 
------------------------ 
    T1  1  6 
    T1  2  1 
    T1  3  1 
    T2  1  6 
    T2  2  1 
    T2  3  1 

我想要做的是通过teacher得到这一结果的平均rating和组它是：

teacher | rating 
------------------------ 
    T1  2.66667 
    T2  2.66667 

，因为如果我做Select avg(rating) from table group by teacher，我得到

teacher | rating 
------------------------ 
    T1  3.5 
    T2  3.5 

Answer 1

你可以这样做：

SELECT teacher,avg(avgrating) as avgrating 
FROM 
(Select teacher, class, avg(rating) avgrating 
from TableName 
group by teacher, class) Temp 
GROUP BY teacher 

结果：

TEACHER AVGRATING 
T1  2.666666 
T2  2.666666 

样品结果SQL Fiddle。

OR：

SELECT T1.teacher,avg(T2.avgrating) as rating 
FROM TableName T1 JOIN 
(SELECT teacher,class,avg(rating) as avgrating 
FROM TableName T2 
GROUP BY teacher,class) T2 ON T1.teacher=T2.teacher 
GROUP BY T1.teacher 

SQL Fiddle。

Answer 2

使用嵌套查询：

select teacher , avg(avgr) from 
(Select teacher, class, avg(rating) avgr from table group by teacher, class) qry 
group by teacher