（165）You have an ORDERS table with the following structure:

The table has data in the ODATE column for all rows. Many orders are placed in a single day.

You need to ensure that the ODATE column must contain data for every order in future.

Which method would serve the purpose?

A. Modify the column using the ALTER TABLE...MODIFY command.

B. Add a UNIQUE constraint to the column using the ALTER TABLE...ADD CONSTRAINT command.

C. Add a NOT NULL constraint to the column using the ALTER TABLE...ADD CONSTRAINT command.

D. Add  a  PRIMARY  KEY  constraint  to  the  column  using  the  ALTER  TABLE...ADD  CONSTRAINT command.

答案：（A）