無效的ID查詢

Question

當我輸入驗證碼我不斷收到關於DepartmentName「無效的標識符」：

SELECT LastName "Last Name", Gender "Gender" 
FROM Employee 
WHERE DepartmentName = 
(SELECT DepartmentName FROM Department WHERE DepartmentName = 'Radiology'); 

任何幫助表示讚賞！

Answer 1

如果在創建表的時候在列名中有雙引號（比如說「DepartmentName」），那麼在選擇查詢中也應該有帶雙引號的列。

如果使用Where DepartmentName，你會得到一個錯誤ORA-00904-無效標識符

因此，爲了避免錯誤，你必須使用WHERE「DepartmentName的」

Answer 2

我要去猜測其意圖是這樣的：

SELECT e.LastName as "Last Name", e.Gender as "Gender" 
FROM Employee e 
WHERE e.DepartmentId = (SELECT d.DepartmentId 
         FROM Department d 
         WHERE d.DepartmentName = 'Radiology' 
         ); 

也就是說，你想這兩個表連接的某種id。如果您在Employee有DepartmentName，你會這樣寫：

SELECT e.LastName as "Last Name", e.Gender as "Gender" 
FROM Employee e 
WHERE e..DepartmentName = 'Radiology'; 

否「加盟」是必要的。

Answer 3

可能性是部門名稱未存儲在employee表中，但部門表中某行的某個「外鍵」是。因此，您可以加入這兩個表格

SELECT e.LastName as "Last Name", e.Gender as "Gender" 
FROM Employee e 
INNER JOIN Department d ON e.DepartmentId = d.id -- guessed this column name 
WHERE d.DepartmentName = 'Radiology' 

如果遇到連接或IN（）的選擇，請使用連接。