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Telephone Directory Example

Figure 15 shows the telephone directory example and includes the entities, relationships, and attributes. This diagram includes the relationships that you establish with the matrix. After you study the diagram symbols, compare the E-R diagram in Figure 15 with the matrix in Figure 10. Verify for yourself that the relationships are the same in both figures.

A matrix such as Figure 10 is a useful tool when you first design your model, because when you fill it out, you are forced to think of every possible relationship. However, the same relationships appear in a diagram such as Figure 15, and this type of diagram might be easier to read when you review an existing model.

Figure 15. Preliminary Entity-Relationship Diagram of the Telephone Directory Example
begin figure description - There are five entities in the diagram. Each entity is represented by a rectangle. The name of each entity is above its rectangle. The attributes of each entity are inside its rectangle. The names of the five entities are: name, voice, fax, modem, and address. The attributes of the "name" entity are: lname, fname, bdate, anniv, email, child1, child2, and child3. The attributes of the "address" entity are: street, city, state, and zipcode. The attributes of the "voice" entity are: vce_num and vce_type. The attributes of the "fax" entity are: fax_num, oper_num, and oper_till. The attributes of the "modem" entity are: mdm_num, b9600, b14400, and b28800. The "name" entity is connected to each of the other four entities by relationships. None of the other four entities are connected to each other. The relationship between "name" and "voice" has no special symbols on the end near "name". The end near "voice" has the symbol for "optional" and the symbol for "many". The relationship between "name" and "fax" has the symbol for "many" on the end near "name". The end near "fax" has the smbols for "optional" and "many". The relationship between "name" and "modem" has the symbol for "exactly one" on the end near "name". The end near "modem" has the symbols for "optional" and "many". The relationship between "name" and "address" has the symbols for "optional" and "many" on the end near "name". The end near "address" has the symbols for "optional" and "exactly one". - end figure description

After the Diagram Is Complete

The rest of this chapter describes how to perform the following tasks:

Implementing a Relational Data Model shows you how to create a database from the E-R data model.

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