在SQL中創建查詢

Question

我試圖創建一個查詢，該查詢將顯示不指導任何其他員工的工作人員的姓名。它也應該以姓氏命令。

到目前爲止，我有這樣的：

SELECT a.name, m.mentor 
FROM accountant AS a 
LEFT OUTER JOIN accountant AS m ON a.mentor = m.staff_id 
WHERE m.mentor = NULL 
ORDER BY m.surname; 

當我運行查詢不返回任何結果。

任何幫助將是很好的。

Answer 1

嘗試使用IS NULL不= NULL

SELECT a.name, m.mentor 
FROM accountant AS a LEFT OUTER JOIN accountant AS m 
ON a.mentor = m.staff_id 
WHERE m.mentor is NULL /// here 
ORDER BY m.surname; 

Answer 2

SELECT a.name, m.mentor 
FROM accountant AS a LEFT OUTER JOIN accountant AS m 
ON a.mentor = m.staff_id 
WHERE m.mentor IS NULL 
ORDER BY m.surname; 

您需要使用IS NULL，你不能檢查，如果該值equals(=)到NULL