A PRIMARY KEY constraint can be added after a table has been created and populated because Oracle allows adding constraints to existing tables. A FOREIGN KEY column can contain NULLs because NULL values do not violate referential integrity. A UNIQUE constraint can use a pre-existing index on the constrained column or columns to enforce uniqueness without creating a new index. A UNIQUE constraint permits NULLs because NULLs are not considered equal to other NULLs, thus they do not violate the uniqueness constraint. A CHECK constraint cannot refer to values in other rows; it only applies to the values in the columns of the row where it is defined. A NOT NULL constraint is defined at the column level, not the table level. A column can have multiple CHECK constraints, not just one.