SQL query: Delete a entry which is not present in a join table?

Question

Hi, I’m going to delete all users which has no subscription but I seem to run into problems each time I try to detect the users.

My schemas look like this:

Users = {userid, name}

Subscriptionoffering = {userid, subscriptionname}

Now, what I’m going to do is to delete all users in the user table there has a count of zero in the subscriptionoffering table. Or said in other words: All users which userid is not present in the subscriptionoffering table. I’ve tried with different queries but with no result.

I’ve tried to say where user.userid <> subscriptionoffering.userid, but that doesn’t seem to work. Do anyone know how to create the correct query?

Thanks

Mestika

Answer 1

delete from Users 
where UserID not in
    (select userid from subscriptionOffering)

Answer 2

You can use a multi-table delete statement with a left outer join and focus on the non-matching rows like this:

delete u from Users as u
left outer join Subscriptionoffering as so
on so.userid = u.userid
where so.userid is null;

Here is some test code to prove it:

mysql> create table Users (userid int unsigned primary key auto_increment) engine = innodb;
Query OK, 0 rows affected (0.43 sec)

mysql> create table Subscriptionoffering (userid int unsigned not null, subscriptionname varchar(32) not null, foreign key (userid) references Users(userid)) engine = innodb;
Query OK, 0 rows affected (0.41 sec)

mysql> insert into Users () values (), (), (), (), ();
Query OK, 5 rows affected (0.38 sec)
Records: 5  Duplicates: 0  Warnings: 0

mysql> select * from Users;
+--------+
| userid |
+--------+
|      1 |
|      2 |
|      3 |
|      4 |
|      5 |
+--------+
5 rows in set (0.00 sec)

mysql> insert into Subscriptionoffering (userid, subscriptionname) values (1, 'One'), (3, 'Three'), (5, 'Five');
Query OK, 3 rows affected (0.31 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> select * from Subscriptionoffering;
+--------+------------------+
| userid | subscriptionname |
+--------+------------------+
|      1 | One              |
|      3 | Three            |
|      5 | Five             |
+--------+------------------+
3 rows in set (0.00 sec)

mysql> delete u from Users as u
    -> left outer join Subscriptionoffering as so
    -> on so.userid = u.userid
    -> where so.userid is null;
Query OK, 2 rows affected (0.36 sec)

mysql> select * from Users;
+--------+
| userid |
+--------+
|      1 |
|      3 |
|      5 |
+--------+
3 rows in set (0.00 sec)