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Identifying Attributes

Entities contain attributes, which are characteristics or modifiers, qualities, amounts, or features. An attribute is a fact or nondecomposable piece of information about an entity. Later, when you represent an entity as a table, its attributes are added to the model as new columns.

You must identify the entities before you can identify the database attributes. After you determine the entities, ask yourself, "What characteristics do I need to know about each entity?" For example, in an address entity, you probably need information about street, city, and zip code. Each of these characteristics of the address entity becomes an attribute.

Selecting Attributes for Entities

To select attributes, choose ones that have the following qualities:

The rules for how to define attributes are the same as those for how to define columns. For information about how to define columns, see Placing Constraints on Columns.

The following attributes are added to the telephone directory example to produce the diagram that Figure 15 shows:

Listing Attributes

For now, list the attributes for the telephone directory example with the entities with which you think they belong. Your list should look like Figure 11.

Figure 11. Attributes for the Telephone Directory Example
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About Entity Occurrences

An additional data object is the entity occurrence. Each row in a table represents a specific, single occurrence of the entity. For example, if customer is an entity, a customer table represents the idea of customer; in it, each row represents one specific customer, such as Sue Smith. Keep in mind that entities become tables, attributes become columns, and entity occurrences become rows.

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