Which of the following statements is true about constraints?

A.
A constraint can be added to an existing table using the ALTER TABLE statement.

B.
Two or more same values can be assigned to the column when there is a unique constraint.

C.
The NOT NULL constraint can be used to ensure that a particular column in a table is never
assigned a zero value

D.
The predefined value assigned by a default constraint cannot be overwritten.

Explanation:
: http://db.apache.org/derby/manuals/reference/sqlj26.html#HDRSII-SQLJ-20863