“The single most important concept in relational schema design theory is that of a functional dependency,” write Ramez Elmasri and Shamkant Navathe in “Fundamentals of Database Systems.”
But what is a functional dependency? It is the actual relationships in the relational database. It is the relations among the attributes within a table. It is a constraint between two sets of attributes. The relations are permanent and unalterable.
For instance, a table may have two attributes, or columns. If one is the primary key, we can then say we can always determine the value of the second attribute using the primary key. This means we can use the primary key as an index to look up the second attribute. It is a mathematical certainty. The primary key is the determinant and the other is the dependent, the parlance of database-speak.
(Keep in mind that this relationship does not work in reverse. You can not use a dependent value to definitively determine the primary key, chiefly because the dependent value may not be unique in a given table).
Beyond the simple connection drawn between the primary key and the dependents, a number of other inferences can be made as well, using Armstrong’s inference rules, which cover the laws of logic such as transitivity and reflexivity.
All material taken from a class I’m attending at UMUC on relational databases, as well as from the book….
…All mistakes are my own, though..