Which three statements are true about defining relations between tables in a relational database?

A. Correct. Foreign key constraints can be nullable. This allows the possibility of a row not having a link to another table. B. Incorrect. Unique key constraints do not allow multiple rows to have null values unless the constraint is defined on multiple columns. C. Incorrect. Primary key columns must be NOT NULL and unique across the table. D. Incorrect. Primary or unique key values do not refer to foreign keys; it's the foreign keys that refer to primary or unique keys. E. Correct. Foreign key constraints enforce referential integrity by requiring that each foreign key value matches a primary or unique key value in the related table.

These are standard rules in relational databases, which can be confirmed in the Oracle Database Concepts Guide.