||Oracle Database Tips by Donald Burleson
Entity Relation Modeling
Data modeling is a method for developing the
structure of the database. It uses various tools to define the data,
relationships, semantics and data consistency requirements. The
Entity Relation Diagram (ERD) is where
most designers start when creating and normalizing a database. An ERD
normally starts on paper with a general layout of the database data
entities and their relationships.
The entity/relation model was first introduced by
Professor Emeritus Peter Chen from the University of Louisiana, and it
is sometimes called a Chen diagram. In the 25
years since the introduction of this model, many permutations have
been created, but the basic principles of entity/relations modeling
While an E/R model may be free of redundant
information, it is impossible to implement the model in a relational
database without introducing redundancy to support the data
relationships. For example, if a data model was implemented using a
pointer-based DBMS, such as IMS, pointers would be used to establish
relationships between entities.
At some point, the diagram becomes unwieldy on
paper and an ERD tool is used to graphically finish the design. In a
nutshell, the ER diagram shows the data tables, and more importantly,
it shows the one-to-many and many-to-many data relationships:
Why start on paper? Because an ERD is so simple
that in the early stages as the design progresses, it is easier to
just redraw it by hand. In the beginning, we are not concerned with
details of a relationship just that a relationship exists. For
example, we are not concerned with what the relationship is between an
employee and a department, just that there is a relationship and that
it is a two-way relationship. As the model continues to be defined,
those relationships become more defined.
One key point to remember is that a relationship
must be defined in both directions. A department may have one or more
employees, but an employee will have zero or one and only one
department. Both rules define a relationship between an employee
entity and a department entity, but they differ significantly
depending on which direction you are discussing. Business rules
eventually will make their way into the later stages of the ERD. First,
let's quickly define the parts of the ERD.
Get the Complete
Oracle SQL Tuning Information
The landmark book
SQL Tuning The Definitive Reference" is
filled with valuable information on Oracle SQL Tuning.
This book includes scripts and tools to hypercharge Oracle 11g
performance and you can
for 30% off directly from the publisher.