Constructing a Logical Model

What’s In This Chapter?

The first step in constructing a logical model is developing the Entity Relationship Diagram (ERD), a high level data model of a wide business area. An entity-relationship diagram is made up of three main building blocks — entities, attributes, and relationships. If we view a diagram as a graphical language for expressing statements about your business, entities are the nouns, attributes are the adjectives or modifiers, and relationships are the verbs. Building a data model with ERwin is simply a matter of finding the right collection of nouns, verbs, and adjectives and putting them all together.

The objective of the ERD is to provide a view of business information requirements sufficient to satisfy the need for broad planning for development of its information system. These models are not very detailed (only major entities are included,) and there is not much detail, if any, on attributes. Many-to-many (non-specific) relationships are allowed, and keys are generally not included. This is primarily a presentation or discussion model.

ERDs are also divided into subject areas, which are used to define “business views” or specific areas of interest to individual business functions. Subject areas help reduct larger models into smaller packages more manageable subsets of entities, that can be more easily defined and maintained.

There are many methods available for developing the ERD. These range from formal modeling sessions (described in the previous chapter) to individual interviews with business managers who have responsibility for wide areas.

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