One refinement we’ll need to make is to resolve the supertype/subtype relationship for the Customer entity. Since relational databases do not support the concept of subtyping, we need to resolve this in our physical data model. Recall from our earlier discussion that there are three ways of resolving subtyping in the physical model: identity, rolling down, and rolling up. In this case, we’ll choose the rolling down method to preserve two distinct entities: Retail Customer and Wholesale Customer. Since in the original design, the intern neglected to include wholesale customers, two separate applications had been built: one to manage retail customers and one for wholesale customers. Although this is not an ideal situation, ...

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