I can definitely see what Prof. Bud meant by saying that ER modeling can get confusing. For some reason with me, if i think about the model too much I confuse myself. The point where I have got confused is about the optional versus mandatory entity type partioning Section 4.4. It says that when there is a double line joining from the supertype to the line joining the subtypes, it means that the supertype must have classified as one of the subtype. However, in the next section when it talks about disjoint versus overlapping, it says that unless there is a 'd' under the '-' , the supertype could be classified as more than one subtype. But what if the relationship is represented by a single line i.e. with an optiontal subtyping and a '-' with a 'd' underneath it. Does that mean that the supertype does not need to be classified to either of the two subtypes? I am really confused with this part of ER modeling. Hope someone can help me get a clearer picture of this.
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