Sam stopped by for coffee. "You know, I forgot one thing."
"What's that?" I asked.
"I'd really like to keep customers' addresses in the database," he said.
"Well, we don't have a database. That's an implementation issue you shouldn't be concerned with," I replied.
"OK, you got me. I want to be able to keep track of a customer's address so that I can send him mail," Sam stated.
"I think we can do that. You can have the staff enter each customer's address off your cards," I said.
"Yeah, but I also want to check whether my staff entered the addresses correctly. If they enter the wrong Zip Code, the post office might send the mail back. That's a waste," he stated emphatically.
"OK, we'll put Zip Code verification into the CD rental system," I said. "I'll look for a Zip Code verification system."
Then I asked, "What should the system do if the Zip Code is incorrect?"
He said, "Correct it," looking at me as if I were asking the dumbest question he had heard in the last year.
I followed it up with, "What should the system do if the Zip Code cannot be found?"
"Then I guess the user must have made a mistake, so we ought to ask him to correct it," he answered.
"What if the address is not in the Zip Code system yet?" I asked.
"Well, maybe you should keep track of that and try verifying it later," Sam replied.
Based on his responses, I came up with the following preliminary work case for the Zip Code verification system. A work case, as you might recall from Chapter 4 ...