Third normal form

Third normal form

1. A relationship is in the third normal form if there are no transitiveness between its attributes and if it is already in 2 FN.

2. Determination of the transitive dependence

   • If the graph used to bring the relationship to the normal normal form is complete in terms of existing transitiveness, an additional graph is not necessary. The graph to convert to the second normal form requires only that all complete and partial dependencies be known

3. Simple transitivity

   • For the unilateral transitive dependence, the independent variable points to the dependent variable, as presented in the figure where B depends on A. The arc between B and Q has been eliminated; the implicit dependence of B on Q is obvious.
   • Multiple transitive.
     • We establish the simple condition that Z is dependent on a transitive form of Q. If there is more than one intermediate dependency variable, the transitive will not be complete until all such variables are specified. That is, although we start with the condition of transitivity, Q ---> Z,

4. Decomposition

   • Given a sub-relation with one or more transitive dependencies, the decomposition consists in splitting the relation into one or more of a sub-relation, where the intermediate variable appears as a dependent variable in one and as an independent variable in the other.
   • Graphic decomposition
     • Given a 2FN graph, we must first select the nodes pointed by the root that are not leaves. They become roots of their own sub-trees, A''and C ''. These sub-trees are eliminated from Q leaving in Q '' only nodes A and C, which are the roots of A ;; and C ''.

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