MySQL查询联接以从两个表中选择不匹配的行

我有两张桌子，如下所示

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我试过这个问题

SELECT users.* FROM users 
WHERE users.username NOT IN(
     SELECT users_approval.username FROM users_approval 
     WHERE users_approval.approved_id = "3B888F52-50BC-11E2-B08B-99E5B2CADDF7"
) AND users.role_type = "student"

得到了下面的结果

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是否有任何方法可以使用JOIN获取与上述相同的结果集

我很感激你的帮助

SELECT users.* 
FROM   users 
       LEFT JOIN users_approval b
          ON users.username = b.username AND
             b.approved_id = "3B888F52-50BC-11E2-B08B-99E5B2CADDF7"
WHERE  users.role_type = "student" AND
       b.approved_id IS NULL

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假设您在第二个表中有user_id作为外键，它可以用来连接，而不是使用username来连接表

awesome。。！我现在对连接的工作方式有了一些想法：P。非常感谢

SELECT users.* FROM users 
WHERE users.username NOT IN(
     SELECT users_approval.username FROM users_approval 
     WHERE users_approval.approved_id = "3B888F52-50BC-11E2-B08B-99E5B2CADDF7"
) AND users.role_type = "student"

user_id   |   username        |  first_name |   role_type
-------------------------------------------------------------
4         |   testuser4       |  testu4     |    student
5         |   testuser5       |  testu5     |    student
-------------------------------------------------------------

SELECT users.* 
FROM   users 
       LEFT JOIN users_approval b
          ON users.username = b.username AND
             b.approved_id = "3B888F52-50BC-11E2-B08B-99E5B2CADDF7"
WHERE  users.role_type = "student" AND
       b.approved_id IS NULL

SELECT 
    users.* 
FROM users as u
LEFT JOIN users_approval as ua ON  ua.id = u.id
WHERE u.role_type = "student" AND ua.approved_id IS NULL