Page 246 - DCAP405_SOFTWARE_ENGINEERING

Unit 14: Data Modeling

5. Although ……………………….. was widely implemented and used, it failed to become Notes
dominant for two main reasons.
6. All attributes of a specific record are listed under an ……………………. type.

7. The degree of a relation is the number of ……………………. in its relation schema.

14.2 Data Objects

Now you know and understand the entities and relationships in your database, which is the
most important part of the relational-database design process. After you determine the entities
and relationships, a method that displays your thought process during database design might
be helpful.

Most data-modeling methods provide some way to graphically display the entities and
relationships.

Did u know? Did IBM also use E-R Diagrams?
Yes, IBM Informix documentation uses the E-R diagram approach that C. R. Bachman
originally developed.
E-R diagrams serve the following purposes. They:

Model the informational requirements of an organization
Identify entities and their relationships
Provide a starting point for data definition (data-flow diagrams)
Provide an excellent source of documentation for application developers and both database
and system administrators
Create a logical design of the database that can be translated into a physical schema
Several different styles of E-R diagrams exist. If you already have a style that you prefer, use
it. Figure 14.3 shows a sample E-R diagram.
Figure 14.3. Symbols of an Entity-Relationship Diagram

In an E-R diagram, a box represents an entity. A line represents the relationships that connect the
entities. In addition, Figure 14.4 shows how you use graphical items to display the following
features of relationships:

A circle across a relationship link indicates optionality in the relationship (zero instances
can occur).

A small bar across a relationship link indicates that exactly one instance of the entity is
associated with another entity (consider the bar to be a 1).
The crow’s-feet represent many in the relationship.

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