中等ICT-DBAS-013
A pupil table stores Class and ClassNo. Neither field is unique on its own, but each pair identifies one pupil. Which key fits?
A. Class alone, because a class name never repeats.
B. A photograph field, because pictures are always unique.
C. A composite primary key made from Class and ClassNo together.✓ 答案
D. No key is possible once any single field repeats.
解說
A composite key uses two or more fields that are unique as a group. Class by itself repeats, so it cannot be the primary key alone.