Which two statements are true regarding constraints?
A.
A foreign key cannot contain null values.
B.
A column with the unique constraint can contain null values.
C.
A constraint is enforced only for the insert operation on a table.
D.
A constraint can be disabled even if the constraint column contains data.
E.
All constraints can be defined at the column level as well as the table level.
Explanation:
B: Any number of rows can include nulls for columns without NOT NULL constraints because nulls are not
considered equal to anything.
D: Constraints can be added, dropped, enabled, disabled, or validated. DISABLE allows incoming data,regardless of whether it conforms to the constraint
Incorrect:
Not A: The relational model permits the value of foreign keys either to match the referenced primary or unique
key value, or be null.
http://docs.oracle.com/cd/B10500_01/server.920/a96524/c22integ.htm