SQL“无效的列名”错误，内部选择

Question

我有以下SQL：

SELECT A.*, 
(SELECT answer FROM [tblAnswers] B 
WHERE  B.memberID = A.memberID AND QuestionID = 3) AS ethnicity
FROM [tblMembers] A  
WHERE 
LOWER(gender) = 'm' 
AND ethnicity = 'Sephardi' 

我得到错误Invalid column name 'ethnicity'指的是对该列的最后引用。

问题：为什么此列不可用于WHERE子句？

我知道，如果我执行以下操作，它将起作用：

SELECT A.*, 
(SELECT answer FROM [tblAnswers] B WHERE  B.memberID = A.memberID AND QuestionID = 3) AS ethnicity
FROM [tblMembers] A  
WHERE
LOWER(gender) = 'm' 
AND convert(nvarchar, (SELECT answer FROM [tblAnswers] B WHERE  B.memberID = A.memberID AND QuestionID = 3)) = 'Sephardi'  

Answer 1

如果不专门引用整个计算（查询，大小写等），则不能在where子句中使用“计算”​​列。

您应该加入查询表，而不是对其进行子查询：

Select A.*, B.answer
From tblMembers A
    Inner Join tblAnswers B on A.memberID = B.membeID and A.QuestionID = 3
Where Lower(gender) = 'm'
    and B.answer = 'Sephardi'

Answer 2

您不能使用在以下WHERE子句中的SELECT子句中刚分配的列名。 通常，您可以将其放在子查询中，然后使用列名：

SELECT * FROM
(
    SELECT A.*, 
    (SELECT answer FROM [tblAnswers] B 
    WHERE  B.memberID = A.memberID AND QuestionID = 3) AS ethnicity
    FROM [tblMembers] A  
    WHERE LOWER(gender) = 'm' 
) s
WHERE ethnicity = 'Sephardi' 

但是，此查询可能更适合于JOIN ：

SELECT A.*, t.answer AS ethnicity
FROM [tblMembers] A  
LEFT JOIN tblAnswers t ON b.memberID = A.memberID AND QuestionID = 3
WHERE LOWER(gender) = 'm' AND t.answer = 'Sephardi'