There is one issue that came to mind when I looked at the ER model we worked on in class today, Exercise 2. When you look at the entity type "Sold to Us" and look at its relation to "Customer" and "Sales Person" you see the famous single hash mark that we discussed for quite a while in class today. My initial question is how this single hash differs from a double hash mark. One thing that seems to work in this case (and a notation that I think would be very useful) is that the single hash implies that the relationship can exist but is not essential that it exists. In this example, a customer does not have to sell a car to exist in the database. If this is the case I am not sure why the notation would not be a "O" followed by a hash mark. I am still a little fuzzy on the purpose of the single hash and its meaning.
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