Oracle Database SQL — Question 215

Which three statements are true about defining relations between tables in a relational database? (Choose three.)

Answer options

• A. Primary key columns allow null values.
• B. Every primary or unique key value must refer to a matching foreign key value.
• C. Every foreign key value must refer to a matching primary or unique key value.
• D. Foreign key columns allow null values.
• E. Unique key columns allow null values.

Correct answer: C, D, E

Explanation

Option C is correct because every foreign key must reference an existing primary or unique key, ensuring data integrity. Option D is also correct as foreign keys can indeed have null values, indicating no relationship in certain cases. Option E is correct because unique key columns can have null values, allowing for flexibility in data entries, while options A and B are incorrect due to the constraints they imply about primary keys and their relationship to foreign keys.