mysql> select topic_id from article_threads where topic_id > 0 and topic_id not in (select id from kids);
Empty set (8.66 sec)

mysql> explain select topic_id from article_threads where topic_id not in (select id from kids);
+----+--------------------+-----------------+-------+---------------+------+---------+------+------+--------------------------+
| id | select_type        | table           | type  | possible_keys | key  | key_len | ref  | rows | Extra                    |
+----+--------------------+-----------------+-------+---------------+------+---------+------+------+--------------------------+
|  1 | PRIMARY            | article_threads | index | NULL          | tid  | 5       | NULL | 4015 | Using where; Using index |
|  2 | DEPENDENT SUBQUERY | kids            | ALL   | NULL          | NULL | NULL    | NULL | 2033 | Using where              |
+----+--------------------+-----------------+-------+---------------+------+---------+------+------+--------------------------+
2 rows in set (0.00 sec)

mysql> select count(*) from article_threads;
+----------+
| count(*) |
+----------+
|     4015 |
+----------+
1 row in set (0.00 sec)

mysql> select count(*) from kids;
+----------+
| count(*) |
+----------+
|     2033 |
+----------+
1 row in set (0.00 sec)

mysql> desc kids;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id    | int(11) | YES  |     | NULL    |       |
+-------+---------+------+-----+---------+-------+
1 row in set (0.00 sec)

mysql> desc article_threads;
+-------------+--------------+------+-----+---------+-------+
| Field       | Type         | Null | Key | Default | Extra |
+-------------+--------------+------+-----+---------+-------+
| topic_id    | int(11)      | YES  | UNI | NULL    |       |
| category_id | int(11)      | YES  | MUL | NULL    |       |
| filename    | varchar(55)  | NO   | PRI |         |       |
| author_name | varchar(100) | YES  |     | NULL    |       |
| statusFlags | varchar(5)   | YES  |     | NULL    |       |
| discussion  | int(11)      | YES  |     | NULL    |       |
| slug        | varchar(255) | YES  | UNI | NULL    |       |
+-------------+--------------+------+-----+---------+-------+
7 rows in set (0.00 sec)