Database Reference
In-Depth Information
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Number of occurrences in related pairs
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Method for identifying each unique occurrence of an entity type
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Special entity types
Data Modeling Aids
In practice, how do you create a semantic or generic data model? You know that
you need symbols to denote each component you want to use to build the model.
The proper arrangement of appropriate symbols constitutes a data model diagram.
A data model diagram gives you a graphic picture of the model. When you look at
the data model diagram, you observe what the objects are, how they are linked
together, and what the characteristics of each object are.
Graphic Tools.
You may use any commercial graphic tool to draw the standard
symbols and connecting lines to produce a data model diagram. This is still a useful
method for creating data model diagrams, but it has limitations. It produces a static
diagram; any revisions must be made by adding, changing, or deleting the symbols.
Moreover, the diagram itself cannot be used directly to proceed to the next phases
of the database development life cycle.
CASE Tools.
Computer-aided software engineering (CASE) tools overcome many
of the limitations of the plain graphic tools for drawing data model diagrams and
creating semantic data models. CASE tools produce the data model diagrams and
also keep the model components stored. Revisions may be made easily. Most CASE
tools enable you to take the model to the next step. You can create the semantic
model and transform it into a conventional data model and then into the physical
model for the target commercial database system. This process is called forward
engineering of the data model. Some CASE tools also have backward engineering
capabilities, with which you can take a physical schema of an Oracle database and
convert it back into a semantic model.
DATA VIEWS
In Chapter 4, while discussing the design phase of DDLC, we referred to the exter-
nal schema. Recall that the external schema relates to the views of the database
content by individual user groups. The external schema depicts the database as user
groups perceive or view it from the outside—an external view.
What are these external perceptions of the database? What do the external per-
ceptions hold? How do the users see the database? What is the relevance of these
external views to data modeling? These external views are also referred to as
data
views
or
user views
.
The concept of data views is critical in the development of a database system. It
forms the underlying basis for creating a data model, which in turn leads us to the
design. Later on, you will also learn about the role of data views in providing secu-
rity to the database system. We will now explore the significance of data views and
their role in data modeling and logical design. We will present different aspects of
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