This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
Primary key ensures uniqueness. But individual columns of a primary key are (in general) not unique. Foreign constraints are required by SQL standard to refer to a unique key. I'm surprised that MySQL even allows his situation.