DBMS-Single-valued Versus Multivalued Attributes
Most attributes have a single value for a particular entity; such attributes are called single-valued. For example, Age is a single-valued attribute of person. In some cases an attribute can have a set of values for the same entity—for example, a Colors attribute for a car, or a CollegeDegrees attribute for a person. Cars with one color have a single value, whereas two-tone cars have two values for Colors. Similarly, one person may not have a college degree, another person may have one, and a third person may have two or more degrees; so different persons can have different numbers of values for theCollegeDegrees attribute. Such attributes are called multivalued. A multivalued attribute may have lower and upper bounds on the number of values allowed for each individual entity. For example, the Colors attribute of a car may have between one and three values, if we assume that a car can have at most three colors.
