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Data Warehouse Entity Relation

Oracle Tips by Burleson Consulting

The Data Warehouse Development Life Cycle

Oracle Data Warehouse Design

De-normalizing 3NF Relationships

Another example is the zip_code attribute in the student entity. At first glance, it appears that a violation of third normal form (that is, a transitive dependency) has occurred between city and zip_code. In other words, it appears that a zip_code is paired with each student's city of residence. Since each city has many zip_codes, and each zip_code refers only to one city, it makes sense to model this as a one-to-many data relationship. The presence of this data relationship requires creating a separate entity called zip with attached student entities. However, this is another case where an entity (zip, in this case) lacks key attributes. It would be impractical to make zip an entity. In other words, zip_code has no associated data items. Creating a database table with only one data column would be nonsense, and the model would finally appear as shown in Figure 4.6.

Figure 4.6 An example of correct many-to-many relationships.

This example demonstrates that it is not enough to group "like" items and then identify the data relationships. A practical test must be made regarding the presence of no-key attributes within an entity class. If an entity has no attributes (that is, the table has only one field), the presence of the entity is nothing more than an index to the foreign key in the member entity, which means that it can be removed from the E/R model. This technique not only simplifies the number of entities, but it creates a better environment for a client/server architecture. More data is logically grouped together, resulting in less data access effort.

This is an excerpt from "High Performance Data Warehousing", copyright 1997.
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