What is a functional dependency?
Once you’ve entered the data items for a subject, you must specify how those items are related to each other. You define these relationships through functional dependencies (FDs). An FD consists of two parts: the determinant, usually the left side; and the determined, usually the right side. Each side is made up of one or more data items. The logic of an FD is as follows: for each instance of the determinant there is exactly one corresponding instance of the determined. In other words, the value of the determinant uniquely determines the value of the determined. The converse is not necessarily true; many determinants may be associated with a single determined. One of the most common FDs occurs when a subject has an ID that uniquely identifies it. Then the data items describing that subject are determined by the ID. For example, every US citizen has a social security number. Other information about a person, such as the name and date of birth, is uniquely identified the social security
