ITPEC FE Morning April 2023 Question 27

Primary Key vs Foreign Key — constraints in a relational database.

• •Primary key (PK): values must be unique and NOT NULL; identifies each row
• •Foreign key (FK): references the primary key of a related table; values can be duplicated (one-to-many) and can be NULL (optional relationship)

Why not others:
- (a) — PK can be composite, but does not have to contain a FK column
- (b) — FK values can be duplicated and can be NULL
- (d) — reversed logic: FK must reference the related table's PK, not the other way around

Key rule: PK = unique + not null; FK = must match an existing PK value (or be NULL), duplicates allowed.

AI-generated — may contain errors