為什么where子句在我的SQL查詢中不起作用？

Question

在下面的代碼中，如果我將where子句替換為where子句，則會收到1054錯誤，其中說where子句中的列tsal未知。 為什么？ 也請給我一些啟發，特別是在必須使用had以及必須使用where子句的情況下。

select employee_id,salary*months as tsal1
from Employee as h
having tsal1=(select max(e.tsal) as metsal
from (select employee_id,salary*months as tsal
from Employee) as e)

Answer 1

MySql 不允許您從where子句中引用別名（在select子句中定義）。

要解決此問題，只需對該別名重復表達式：

select employee_id, salary*months as tsal1
from   Employee as h
where salary*months=(select max(e.tsal) as metsal
                     from   (select employee_id,salary*months as tsal
                             from   Employee) as e)

注意：不必有兩個內部子查詢。 您可以簡化為：

select employee_id, salary*months as tsal1
from   Employee as h
where  salary*months=(select max(salary*months) from Employee)