Chapter 4 - Normalizing a Data Model

Posted by Addison Wesley Free Book | C# Language September 01, 2009
Data normalization is probably one of the most talked-about aspects of database modeling. Before building your data model, you must answer a few questions about normalization. These questions include whether or not to use the formal normalization forms, which of these forms to use, and when to denormalize.

Third Normal Form (3NF)
Third normal form is the form that most well-designed databases meet. 3NF extends 2NF to include the elimination of transitive dependencies. Transitive dependencies are dependencies that arise from a non-key attribute relying on another non-key attribute that relies on the primary key. In other words, if there is an attribute that doesn't rely on the primary key but does rely on another attribute, then the first attribute has a transitive dependency. As with 2NF, to resolve this issue we might simply move the offending attribute to a new entity. Coincidentally, in solving the 2NF problem in Table 4.7, we also created a 3NF entity. In this particular case, AgentPrimaryPhone and AgentSecondaryPhone are not actually attributes of an artist; they are attributes of an agent. Storing them in the Artists entity created a transitive dependency, violating 3NF.
The differences between 2NF and 3NF are very subtle. 2NF deals with partial dependency, and 3NF with transitive dependency. Basically, a partial dependency means that attributes in the entity don't rely entirely on the primary key. Transitive dependency means that attributes in the entity don't rely on the primary key at all, but they do rely on another non-key attribute in the table. In either case, removing the offending attribute (and related attributes, in the 3NF case) to another entity solves the problem.
One of the simplest ways to remember the basics of 3NF is the popular phrase, "The key, the whole key, and nothing but the key." Because the normal forms are nested, the phrase means that 1NF is met because there is a primary key ("the key"), 2NF is met because all attributes in the table rely on all the attributes in the primary key ("the whole key"), and 3NF is met because none of the non-key attributes in the entity relies on any other non-key attributes ("nothing but the key"). Often, people append the phrase, "So help me Codd." Whatever helps you keep it straight.
Boyce-Codd Normal Form (BCNF)
In certain situations, you may discover that an entity has more than one potential, or candidate, primary key (single or composite). Boyce-Codd normal form simply adds a requirement, on top of 3NF, that states that if any entity has more than one possible primary key, then the entity should be split into multiple entities to separate the primary key attributes. For the vast majority of databases, solving the problem of 3NF actually solves this problem as well, because identifying the attribute that has a transitive dependency also tends to reveal the candidate key for the new entity being created. However, strictly speaking, the original 3NF definition did not specify this requirement, so BCNF was added to the list of normal forms to ensure that this was covered.

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