Question: When you say the E/R diagram should have "in the range of five or so entity sets, and a similar number of relationship sets", what exactly do you mean?
Answer: Shoot for somewhere between 3 and 10. You'll sense if your design is too simple or too complex.
Question: What kind of data will I be loading into my database?
Answer: You'll actually create two databases, a small one (10's of entities/relationships) and a large one (1000's or 10,000's of entities/relationships). Many students find that it only makes sense for their small database to contain realistic data, while the large one is made up of synthetic (computer-generated fake) data. If you have an application where you can get a large amount of real data, all the better, but it's not necessary.
Question: I'm completely at a loss to think of an application. Any suggestions?
Answer: Take a look at any Web shopping site. They all have a similar theme: products, customers, orders, shopping baskets, etc. They typically make for an interesting and appropriately sized application.
Question: Written assignment #1 asks me to create a schema for a Web site of my choosing. I'm doing essentially the same thing for my PDA. Can I turn in the same solution for both?
Answer: Yes, no problem. Be sure to follow the instructions in each assignment carefully though.
Question: I have an entity set that has multiple keys. (For example, in entity set Student, it could be that (name,address) is one key and SS# is another.) What should I do?
Answer: Multiple keys do occur in practice. In an entity set in the E/R model there is no way to specify multiple keys: you can only underline one set of attributes as the key. If there are other keys that you would like to mention, make a separate note. When you get around to creating actual relations in an actual DBMS, there is a way to specify more than one key.