Which two statements are true regarding constraints?

Foreign key columns can contain NULL values, which simply indicate the absence of a relationship to the parent table - only non-null foreign key values must match an existing primary key in the referenced table.

The UNIQUE constraint prevents duplicate non-null values but permits multiple NULL values because NULL is not considered equal to any value, including another NULL, under standard SQL semantics. Oracle enforces this behavior consistently, so a UNIQUE column may have many rows with NULL without violating the constraint.

Constraints are enforced for all DML operations - INSERT, UPDATE, and DELETE - not only for INSERT, since an UPDATE could introduce a duplicate or a DELETE could orphan child rows.

Oracle's ALTER TABLE ... DISABLE CONSTRAINT command suspends constraint enforcement without requiring the column data to be removed or modified, allowing constraints to be disabled regardless of existing data.

The NOT NULL constraint can only be defined at the column level and cannot be specified as a table-level constraint, so not all constraint types support both definition styles.