[英]MySQL Query Join to select unmatched rows from two tables
我有两张桌子如下。
user_id | username | first_name | role_type
-----------------------------------------------------------
1 | testuser1 | testu1 | student
2 | testuser2 | testu2 | student
3 | testuser3 | testu3 | student
4 | testuser4 | testu4 | student
5 | testuser5 | testu5 | student
6 | testuser6 | testu6 | admin
7 | testuser7 | testu7 | admin
-----------------------------------------------------------
user_id | username | approved_id
----------------------------------------------------------------------
1 | testuser1 | 3B888F52-50BC-11E2-B08B-99E5B2CADDF7
2 | testuser2 | 3B888F52-50BC-11E2-B08B-99E5B2CADDF7
3 | testuser3 | 3B888F52-50BC-11E2-B08B-99E5B2CADDF7
----------------------------------------------------------------------
我试过了这个问题
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"
并得到以下结果
user_id | username | first_name | role_type
-------------------------------------------------------------
4 | testuser4 | testu4 | student
5 | testuser5 | testu5 | student
-------------------------------------------------------------
有没有办法使用JOIN来获取与上面相同的结果集?
帮助很多。
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
假设您在第二个表中将user_id作为外键,则可以使用它来加入而不是使用用户名来连接表
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