mysql> desc a;
+-------+---------+------+-----+---------+----------------+
| Field | Type    | Null | Key | Default | Extra          |
+-------+---------+------+-----+---------+----------------+
| i     | int(11) | NO   | PRI | NULL    | auto_increment |
| k     | int(11) | YES  |     | NULL    |                |
+-------+---------+------+-----+---------+----------------+
2 rows in set (0.00 sec)

mysql> insert into a(k) values ( ifnull((select max(aa.k)+1 from a aa),1) );
Query OK, 1 row affected (0.00 sec)

mysql> insert into a(k) values ( ifnull((select max(aa.k)+1 from a aa),1) );
Query OK, 1 row affected (0.00 sec)

mysql> select * from a;
+---+------+
| i | k    |
+---+------+
| 1 |    1 |
| 2 |    2 |
+---+------+
2 rows in set (0.00 sec)

mysql>