选择平均FROM选择平均

Question

我只是不叩头如何使它工作，我如何可以选择从其他选择

select avg(avg(valoare)) from (select avg(valoare) from note 
where nr_matricol=111 group by ID_CURS) 

我也试过

select avg(alias) from (select avg(valoare) as "alias" from note 
    where nr_matricol=111 group by ID_CURS) 

Answer 1

第二个查询的平均基本上就是你想要的。但是，一旦您开始在SQL中使用双引号标识符，这些标识符就会区分大小写："alias"是与alias不同的名称（因为alias与ALIAS相同）。

所以你需要在整个查询中使用双引号：

select avg("alias") 
from (
    select avg(valoare) as "alias" 
    from note 
    where nr_matricol=111 group by ID_CURS 
) 

另一种选择是使用不需要引用一个名字：

select avg(avg_valoare) 
from (
    select avg(valoare) as avg_valoare 
    from note 
    where nr_matricol=111 group by ID_CURS 
) 

---

虽然不是必需的Oracle为派生表提供别名也是很好的编码风格。

select avg("alias") 
from (
    select avg(valoare) as "alias" 
    from note 
    where nr_matricol=111 group by ID_CURS 
) x --<<< here 

注意，Oracle不会不支持AS关键字为表的别名，所以你不能派生表的别名使用) as x。

Answer 2

您可以使用下面的查询。

select avg(alias) from 
     (select avg(valoare) as alias from note 
     where nr_matricol=111 group by ID_CURS) X 

OR

select avg(avg(valoare)) as alias from note 
    where nr_matricol=111 group by ID_CURS X