A structured type is a user-defined data type that has a structure that is defined in the database. It contains a sequence of named attributes, each of which has a data type. A structured type also includes a set of method specifications.
A structured type may be used as the type of a table, view, or column. When used as a type for a table or view, that table or view is known as a typed table or typed view, respectively. For typed tables and typed views, the names and data types of the attributes of the structured type become the names and data types of the columns of this typed table or typed view. Rows of the typed table or typed view can be thought of as a representation of instances of the structured type. When used as a data type for a column, the column contains values of that structured type (or values of any of that type's subtypes, as defined below). Methods are used to retrieve or manipulate attributes of a structured column object.
Terminology: A supertype is a structured type for which other structured types, called subtypes, have been defined. A subtype inherits all the attributes and methods of its supertype and may have additional attributes and methods defined. The set of structured types that are related to a common supertype is called a type hierarchy and the type that does not have any supertype is called the root type of the type hierarchy.
The term subtype applies to a user-defined structured type and all user-defined structured types that are below it in the type hierarchy. Therefore, a subtype of a structured type T is T and all structured types below T in the hierarchy. A proper subtype of a structured type T is a structured type below T in the type hierarchy.
There are restrictions on having recursive type definitions in a type hierarchy. For this reason, it is necessary to develop a shorthand way of referring to the specific type of recursive definitions that are allowed. The following definitions are used:
A type may not be defined so that one of its attribute types directly or indirectly uses itself. If it is necessary to have such a configuration, consider using a reference as the attribute. For example, with structured type attributes, there cannot be an instance of "employee" with an attribute of "manager" when "manager" is of type "employee". There can, however, be an attribute of "manager" with a type of REF(employee).
A type cannot be dropped when certain other objects use the type, either directly or indirectly. For example, a type cannot be dropped if a table or view column makes a direct or indirect use of the type. See Table 27 for all objects that could restrict the dropping of types.
Structured type column values are subject to the restrictions that apply to CLOB values as specified in Restrictions Using Varying-Length Character Strings.