LifeinAsia - 11:15 pm on Mar 15, 2013 (gmt 0)
Are you sure it says 2 primary keys and not 2 indexes?
got two primary keys
Is my understanding of this correct?
Correct. If you're having trouble visualizing things, go back to the Venn diagrams [en.wikipedia.org] that you probably learned in school and wondered how on Earth you would ever use them again in the real world. :) Working with DBs is basically set theory at its core (think of each table as a separate set of objects).