Question

If I have this table:

+------+-------+---------------+--------+-----------------+------------+-----------+----------------+------+------+--------+------------+------------+
| type | class | username      | userid | userip          | usermobile | useremail | daysleft| pin1 | pin2 | pin3 | active | schoolname | schoolsite |
+------+-------+---------------+--------+-----------------+------------+-----------+----------------+------+------+--------+------------+------------+
| B    | A     | sebbetest     |   1000 | 123.123.123.123 | none       | none      |       50| 0    | 0    | 0    | Y      | none       | none       | 
| A    | A     | stackowerflow |   5355 | 123.123.123.123 | none       | none      |       50| 0    | 0    | 0    | Y      | none       | haha       | 
| C    | A     | good          |   4223 | 123.123.123.124 | none       | none      |       50| 0    | 0    | 0    | Y      | none       | haha       | 
| A    | A     | tester        |   6353 | 123.123.123.125 | none       | none      |       50| 0    | 0    | 0    | Y      | none       | haha       | 
| B    | A     | admin         |   3453 | 123.123.123.125 | none       | none      |       50| 0    | 0    | 0    | Y      | none       | eeee       | 
| A    | A     | sebastian     |   1342 | 123.123.123.126 | none       | none      |       50| 0    | 0    | 0    | Y      | none       | eeee       | 
| C    | A     | username      |   6456 | 123.123.123.125 | none       | none      |       50| 0    | 0    | 0    | Y      | none       | woooooow   | 
+------+-------+---------------+--------+-----------------+------------+-----------+----------------+------+------+--------+------------+------------+

As you see, the user “good” with the IP “123.123.123.124” AND user “sebastian” with IP “123.123.123.126” has no “companions”, no other users on the same IP.

The user “sebbetest” has a companion “stackowerflow”.

The user “tester” has 2 companions: “admin” and “username”.

Now I want to delete these users that lack companions. How I do? For atomic reasons and to prevent concurrent access from munging the database, I would want to delete all “non-companion” users in a single expression of SQL.

I tried with:
DELETE FROM lan WHERE COUNT(userip) = 1;

got this:
ERROR 1111 (HY000): Invalid use of group function

No rows are duplicates.
If there is a need to check if a entry is unique regardless of IP, (type, userid) is unique.

In other words, if IP is unique in a row, delete it.

Answer 1

Use:

DELETE FROM lan
 WHERE userip IN (SELECT x.userip
                   FROM (SELECT yt.userip
                           FROM lan yt
                       GROUP BY yt.userip
                         HAVING COUNT(*) = 1) x )

You get the error because you can’t use COUNT, or any other aggregate functions in the WHERE clause while outside of a subquery. Only in the HAVING clause can you reference aggregate functions in that manner.

It’s possible this might work:

  DELETE FROM lan 
GROUP BY userip
  HAVING COUNT(*) = 1;

Warning

With every DELETE statement, check and double check that the statement will select what you want for deletion. If you are using InnoDB tables, wrap the DELETE in a transaction so you can use ROLLBACK if necessary.