SQL Server中，重复的子查询和别名

Question

我有一个简单的查询，如下所示：

Select SUBSTRING(Email, Charindex('@', Email) + 1, 
    Len(Email) - CharIndex('@', Email)) as EmailDomain, 
    Count(Email) as Total 
from tblPerson 
Group by SUBSTRING(Email, Charindex('@', Email) + 1, 
Len(Email) - CharIndex('@', Email)) 

这是除了有一个子查询重复做工精细：

SUBSTRING(Email, Charindex('@', Email) + 1, 
Len(Email) - CharIndex('@', Email)) 

我已经把一个别名子查询有as EmailDomain，但是当我查询是这样的：

Select SUBSTRING(Email, Charindex('@', Email) + 1, 
    Len(Email) - CharIndex('@', Email)) as EmailDomain, 
    Count(Email) as Total 
from tblPerson 
Group by EmailDomain 

我得到了一个错误：

Invalid column name 'EmailDomain'

如何解决此问题，但同时也不使用重复查询？

Answer 1

答案很简单，您不能在WHERE子句或GROUP BY子句中使用列别名。

你可以尝试这样的：

SELECT EmailDomain, 
     Total 
FROM (
     Select SUBSTRING(Email, Charindex('@', Email) + 1, 
     Len(Email) - CharIndex('@', Email)) as EmailDomain, 
     Count(Email) as Total 
     from tblPerson 
     ) a 
Group by EmailDomain 

Answer 2

在GROUP BY不能使用别名。为了解决这个问题，你可以使用子查询：

SELECT 
    t.EmailDomain, 
    COUNT(t.Email) AS Total 
FROM (
    SELECT 
     SUBSTRING(Email, CHARINDEX('@', Email) + 1, LEN(Email) - CHARINDEX('@', Email)) AS EmailDomain, 
     Email 
    FROM tblPerson 
) t 
GROUP BY t.EmailDomain 

Answer 3

下面SCRIPT TRY：

Select EmailDomain from (Select SUBSTRING(Email, Charindex('@', Email) + 1, 
    Len(Email) - CharIndex('@', Email)) as EmailDomain, 
    Count(Email) as Total 
from tblPerson) tperson 
Group by EmailDomain