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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.
Correct Answer: BD
Explanation/Reference:
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.
Reference: http://docs.oracle.com/cd/B10500_01/server.920/a96524/c22integ.htm
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