Which three statements are true about defining relations between tables in a relational database?
A. Primary key columns allow null values.
B. Every primary or unique key value must refer to a matching foreign key value.
C. Foreign key columns allow null values.
D. Every foreign key value must refer to a matching primary or unique key value.
E. Unique key columns allow null values.
CDE
CDE
CDE
CDE
CDE
CDE