How Much Normalization Is Enough?

From a formal normalization perspective (what an algorithm would find solely from the “shape” of the model, without understanding the meanings of the entities and attributes) there is nothing wrong with the EMPLOYEE- CHILD-SPOUSE model. But just because it is normalized does not mean that the model is complete or correct. It still may not be able to store all of the information that is needed, or may store it inefficiently. With experience, you can learn to detect and remove additional design flaws even after the “pure normalization” is finished.

In the earlier EMPLOYEE-CHILD-SPOUSE model example, you have already discovered that there is no way of recording a CHILD whose parents are both EMPLOYEEs. So you can make additional changes to try to accommodate this type of data.

If you noticed that EMPLOYEE, SPOUSE, and CHILD all represent instances of people, you may want to try to combine the information into a single table that represents facts about people and one that represents facts about relationships. To fix the model, you can eliminate CHILD and SPOUSE, replacing them with PERSON and PERSON-ASSOCIATION. This lets you record parentage and marriage through the relationships between two PERSONs captured in the PERSON-ASSOCIATION entity.

EMPLOYEE, PERSON, and PERSON-ASSOCIATION Entities

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In this structure, we can finally record any number of relationships between two PERSONs, as well as a number of relationships we could not record before. For example, the previous model had not considered “adoption.” The new structure, however, automatically covers it. To represent “adoption” you can add a new value to the “person-association-type” validation rule to represent adopted parentage. You can also add “legal guardian,” “significant other,” or other relationships between two PERSONs later, if needed.

EMPLOYEE remains an independent entity, because the business chooses to identify EMPLOYEEs differently from PERSONs. However, EMPLOYEE inherits the properties of PERSON by virtue of the “is a” relationship back to PERSON. Notice the “Z” on that relationship and the absence of a diamond. This is a one-to-zero or one relationship that can sometimes be used in place of a subtype when the subtype entities require different keys. In our example, a PERSON either “is an” EMPLOYEE or not.

If we had wanted to use the same key for both PERSON and EMPLOYEE we could have “encased” the EMPLOYEE entity into PERSON and allowed its attributes to be NULL whenever the PERSON was not an EMPLOYEE. We still could have specified that the business wanted to look up “employees” by a separate identifier, but the business statements would have been a bit different. This structure is shown below.

Generalized Employee Model Construct

Conclusions

What this all basically comes down to in the end is that a model may “normalize,” but may still not be a correct representation of the business. Formal normalization is important. Verifying, perhaps with sets of sample instance tables as we have done here, that the model means something is no less important.

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