Question

我正在尝试创建一个查询，显示不指导任何其他员工的员工姓名。它也应该由姓氏订购。

到目前为止，我已经有了这个：

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;

当我运行查询时，它不会返回任何结果。

任何帮助都会很好。

Answer 1

尝试使用IS Null Not = 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，无法检查值equals(=)是否为NULL

在SQL中创建查询

2 个答案: