B.3.1: Discriminator Attributes

It is common for designers to introduce or discover an attribute such that its value can be used to explicitly determine the subtype an entity belongs to. For example, the item entity type can have an attribute, say itemType, which can have a value from the domain

{“video”, “magazine”, “book”}. When this is done the diagram must include the attribute of course, but additionally the attribute is shown as a discriminator attribute for subtyping purposes and the pertinent value for discriminating shown as well.

Below you will see how these are laid out above and below the connection symbol:

This works well for disjoint subtyping but not necessarily for overlapping subtypes. When overlap is possible a designer may include a discriminator for each subtype, and so there are as many discriminator attributes as there are subtypes. Typically, this is a boolean-valued attribute.