Slide 16.27: Value and object types

SQL3 (Cont.)

Value and Object Types
It is also know as the Major Object-Oriented SQL Extensions:

Extension of the current SQL type system to include ADTs.

In an abstract data type (ADT) structure declaration, the type constructors LIST, SET, and MULTISET can be employed.

Specialization of types such that each subtype has exactly one “maximal” supertype which itself has no other supertype.

Table hierarchies as a form of specialization for relations; that is, a table can be defined as a subtable of one or more other tables.

User-defined functions as part of an ADT. Functions may be realized either in SQL or as external functions.

Method and function calls inside select-expression.

User-defined types are divided into value and object types:

Value type: It acts as a generalization of the SQL2 domain concept. The values of value types behave like values of the predefined types of this languages; that is, they “exist permanently” and need not be explicitly created or destroyed.

(Value-type example) Complex objects without identity can be constructed with the help of value types.

CREATE VALUE TYPE ottoengine (
  hp  INT,
  cc  INT );

CREATE VALUE TYPE vehicledrive (
  engine   ottoengine,
  gearing  VARCHAR(12) );

CREATE TABLE automobile (
  drive    vehicledrive,
  carbody  VARCHAR(20) );

Object type: It supports objects with a value-independent identity. Instances of object type have a well-defined lifetime, must be created explicitly and can cease to exist.