ASA SQL User's Guide
Ensuring Data Integrity
Using column defaults
You can change or remove column defaults using the same form of the ALTER TABLE statement you used to create defaults. The following statement changes the default value of a column named order_date from its current setting to CURRENT DATE:
ALTER TABLE sales_order MODIFY order_date DEFAULT CURRENT DATE
You can remove column defaults by modifying them to be NULL. The following statement removes the default from the order_date column:
ALTER TABLE sales_order MODIFY order_date DEFAULT NULL