The problem here is that there might be many green watermelons that came from a 15 foot vine and weighed 1.25 pounds. To fully specify an instance there must be some way to uniquely identify each watermelon to distinguish between those with identical characteristics. In GDB this is done using an ID that we call an accession number. This is further described in overview section devoted to GDB's data model . There can be one or more values for list attributes.
Tables are complex attributes with related sub-components.
21205 reference Reference attributes are those which point to another object in the database. If we expand the example model to have a new class, Orchard , then Apple can have the reference attribute from orchard with which we can refer to an instance of orchard. This is an especially useful type of attribute and is described in the context of querying and editing in the section on the GDB classes . Some attributes values may be determined based on other attributes in that object or another related object. These attributes are called derived attributes and are only of concern to those wanting to insert or edit objects.