MySQL - 查询没有约会的所有用户

Question

如果我有两个表：用户和约会。我如何查询数据库以查找如下内容：

SELECT * FROM users WHERE (none of: appointments.user = user.id)

我假设我需要某种类型的约会与约会表，只是不知道从哪里开始。

Answer 1

SELECT * FROM users 
LEFT JOIN Appointments ON Users.UserID=Appointments.UserID
WHERE Appointments.UserID is null

Answer 2

试试这个：

SELECT * FROM users WHERE users.id NOT IN (SELECT user FROM appointments)