Read the Introduction and complete exercises 1 through 13 of the Learn SQL the Hard Way book. Don't worry, it's actually not that hard. =) These exercises use SQLite, a simpler database package than PostgreSQL that stores each database as a single file. You already have SQLite installed on your Macs – type sqlite3 in a terminal and you should see a SQLite command prompt (type .quit to exit).
Fork this repository and use it to store any .sql and .db files you create. Make a commit after completing each exercise. Since the base exercises just involve copying commands from the book, attempt to complete all of the extra credit and place any statements you write in the .sql file for that exercise. These should be very doable from what we've already learned in class. When finished, submit a pull request.