here is some additional info:

mysql> desc t1 ;
+-------+--------------+------+-----+------------+-------+
| Field | Type         | Null | Key | Default    | Extra |
+-------+--------------+------+-----+------------+-------+
| a     | varchar(100) | YES  |     | TEST TABLE |       |
| b     | int(11)      | YES  |     | 10         |       |
+-------+--------------+------+-----+------------+-------+
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so they can be NULL!