Object Comparison

SQL performs comparison operations on objects. Comparisons can be explicit, using the comparison operators (=, <, >, <>, <=, >=, !=) and the BETWEEN and IN predicates. Comparisons can be implicit, as in the GROUP BY, ORDER BY, DISTINCT, and UNIQUE clauses.

Comparison of objects makes use of special member functions of the object type: map methods and order methods. To perform object comparison, you must implement either a map method or order method in the CREATE TYPE and CREATE TYPE BODY statements.

For example, the type body for the DataStream type, implements the map member function for DataStream comparison as:

MAP MEMBER FUNCTION DataStreamToInt return integer is 
      c integer := id; 
      begin return c; end; 

This definition of the map member function relies on the presence of the id attribute of the DataStream type to map instances to integers. Whenever a comparison operation is required between objects of type DataStream, the map function DataStreamToInt () is called implicitly by the system.

The object type rational_type does not have a simple id attribute like that for DataStream. For rational_type, the map member function is slightly more complicated. Because a map function can return any of the built-in types; rational_type can return a value or type REAL:

MAP MEMBER FUNCTION RationalToReal RETURN REAL IS
     BEGIN
         RETURN numerator/denominator;
     END;
...

If you have not defined a map or order function for an object type, only equality comparisons are allowed on objects of that type. Oracle SQL performs the comparison by doing a field-by-field comparison of the attributes of that type.