NULL or NOT NULL

In mysql table definiations, how NULL and NOT NULL effects behavior?

AFTER MySQL 5.0.2. If there’s no NOT NULL in the column defination, MySQL add a DEFAULT NULL to defination. If there’s NOT NULL in the defination, in strict mode, an insertion that lacks the column will raise an error, while implict default value will be inserted in non-strict mode.

So for portability, always add DEFAULT for NOT NULL column.

Detail can be found here.