Below is a list of a few ER Modelling related questions I have. If anyone can answer any of the following, it would be very helpful to me.
1) In the coursepack, when talking about subtypes, it mentions "optional subtyping is represented by leaving a single line from the supertype to the circle." My question is, what is "circle" referring to?
2) Are subtypes necessary or these entites always abe to be represented as regular entity types with additional relationships attached to them?
3) Referring to the ER end of chapter questions in the coursepack, could the answer to question 3, part 2 be drawn up using a ternary relationship to express the relatoinship between employee, skill and machine type? It is not shown this way in the chapter answers, but I thought it was a possibility.
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