(root@localhost) [test]> create temporary table test1 (name varchar(10), num int unsigned);
Query OK, 0 rows affected (0.26 sec)

(root@localhost) [test]> show create table test1;
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                     |
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------+
| test1 | CREATE TEMPORARY TABLE `test1` (
  `name` varchar(10) default NULL,
  `num` int(10) unsigned default NULL
) ENGINE=MyISAM DEFAULT CHARSET=latin1 | 
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

(root@localhost) [test]> insert into test1 (name) values ('num=?');
Query OK, 1 row affected (0.01 sec)

(root@localhost) [test]> select * from test1;
+-------+------+
| name  | num  |
+-------+------+
| num=? | NULL | 
+-------+------+
1 row in set (0.00 sec)

(root@localhost) [test]> update test1 set num = num + 1;      
Query OK, 0 rows affected (0.00 sec)
Rows matched: 1  Changed: 0  Warnings: 0

(root@localhost) [test]> select * from test1;
+-------+------+
| name  | num  |
+-------+------+
| num=? | NULL | 
+-------+------+
1 row in set (0.00 sec)

(root@localhost) [test]> create temporary table test2 (name varchar(10), num int unsigned not null default 0);
Query OK, 0 rows affected (0.01 sec)

(root@localhost) [test]> show create table test1;
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                     |
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------+
| test1 | CREATE TEMPORARY TABLE `test1` (
  `name` varchar(10) default NULL,
  `num` int(10) unsigned default NULL
) ENGINE=MyISAM DEFAULT CHARSET=latin1 | 
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

(root@localhost) [test]> show create table test2;
+-------+----------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                             |
+-------+----------------------------------------------------------------------------------------------------------------------------------------------------------+
| test2 | CREATE TEMPORARY TABLE `test2` (
  `name` varchar(10) default NULL,
  `num` int(10) unsigned NOT NULL default '0'
) ENGINE=MyISAM DEFAULT CHARSET=latin1 | 
+-------+----------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

(root@localhost) [test]> insert into test2 (name) values ('watch num');
Query OK, 1 row affected (0.00 sec)

(root@localhost) [test]> select * from test2;
+-----------+-----+
| name      | num |
+-----------+-----+
| watch num |   0 | 
+-----------+-----+
1 row in set (0.00 sec)

(root@localhost) [test]> update test2 set num = num+1;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

(root@localhost) [test]> select * from test2;
+-----------+-----+
| name      | num |
+-----------+-----+
| watch num |   1 | 
+-----------+-----+
1 row in set (0.00 sec)

(root@localhost) [test]> update test2 set num = num+1;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

(root@localhost) [test]> select * from test2;
+-----------+-----+
| name      | num |
+-----------+-----+
| watch num |   2 | 
+-----------+-----+
1 row in set (0.01 sec)

(root@localhost) [test]> update test2 set num = num+1;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

(root@localhost) [test]> select * from test2;
+-----------+-----+
| name      | num |
+-----------+-----+
| watch num |   3 | 
+-----------+-----+
1 row in set (0.00 sec)