Computer Science for IGCSE & O level - Databases (Section 5 - No. 2)
Which of the following statements are true about a primary key?
It must be unique.
It can contain null values.
It uniquely identifies each row in a table.
It can be a composite key (multiple columns).
Обяснение
A primary key must be unique, not null, and it identifies each row. It can also consist of multiple columns (composite key).