SQL:2003 allows the definition of user-defined types (UDTs), which we have previously
referred to as abstract data types (ADTs). They may be used in the same way as the
predefined types (for example, CHAR, INT, FLOAT). UDTs are subdivided into two
categories: distinct types and structured types. The simpler type of UDT is the distinct
type, which allows differentiation between the same underlying base types. For example,
we could create the following two distinct types: