Database Reference
In-Depth Information
5.2.5 The Union Rule
The union rule is the reverse of the decomposition rule in that if X Y and X
Z, then X YZ.
5.2.6 Full functional dependence
Some times X determines Y, but X combined with Z does not determine Y. Y
depends on X and X alone. If Y depends on X with anything else, there is not full functional
dependence. Essentially X, the determinant, cannot be a composite key. A composite
key contains more than one field (the equivalent of X with Z).
5.2.7 Multiple valued dependency
Not all dependencies can be modeled in terms of functions. Y is said to be nonfunctionally
dependent on data-item X if for every value of data-item X there is a
delimited set of values for data-item Y. This is also known as a non-functional dependency.
A commonly used example of a multi-valued dependency is a field containing a commade-
limited list or collection of some kind. A collection could be an array of values of the
same type. Those multiple values are dependent as a whole on the primary key, as a
whole meaning the entire collection in the comma delimited list. A trivial multi-valued
dependency occurs between two fields when they are the only two fields in the table.
