Choose three.
Which three statements are true about defining relations between tables in a relatonal database?
A) Foreign key columns allow null values.
B) Unique key columns allow null values
C) Primary key columns allow null values.
D) Every primary or unique key value must refer to a matching foreign key value.
E) Every foreign key value must refer to a matching primary or unique key value.
Answer：ABE
(解析：AB Answer：比较简单明了;D Answer：说反了)