Data Modeling with the Entity-Relationship Model - Database Processing: Fundamentals, Design, and Implementation - page 220

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Under what conditions do you prefer the model in A to that in B?

C.

Under what conditions do you prefer the model in B to that in A? D.

5.57 Examine the list of e-mail messages in Figure 5-54. Using the structure and example
data items in this list, do the following:

Create a single-entity data model for this list. Specify the identifier and all entities. a.

Modify your answer to A to include entities SENDER and SUBJECT. Specify the
identifiers and attributes of entities and the type and cardinalities of the relation-
ships. Explain which cardinalities can be inferred from Figure 5-54 and which need
to be checked out with users.

B.

The e-mail address in the From column in Figure 5-54 is in two different styles. One
style has the true e-mail address; the second style (e.g., Tom Cooper) is the name of
an entry in the user's e-mail directory. Create two categories of SENDER based on
these two styles. Specify identifiers and attributes.

C.

Figure 5-54
Email List