Map and Order Methods:
The values of the scalar datatype like CHAR or REAL have a predefined order that allows them to be compared. While, the instances of an object type has no predefined order. To put them in order, the PL/SQL calls a map method supplied by you.
In the illustration below, the keyword MAP indicates that the method converts orders rational objects by mapping them to the REAL values:
CREATE TYPE Rational AS OBJECT (
num INTEGER,
den INTEGER,
MAP MEMBER FUNCTION convert RETURN REAL,
...
);
CREATE TYPE BODY Rational AS
MAP MEMBER FUNCTION convert RETURN REAL IS
BEGIN
RETURN num / den;
END convert;
...
END;
The PL/SQL uses the ordering to compute the Boolean expressions like x > y, and to do comparisons implied by the GROUP BY, DISTINCT, and ORDER BY clauses. The Map method convert returns to the relative position of an object in the ordering of all the rational objects.
An object type can have only one map method that should be a parameter less function with one of the scalar return types shown below: DATE, VARCHAR2, NUMBER, or an ANSI SQL type like CHARACTER or REAL.
On the other hand, you can supply the PL/SQL with an order method. An object type can have only one order method that should be a function which returns a numeric result. In the illustration below, the keyword ORDER indicates that method match compares 2 objects:
CREATE TYPE Customer AS OBJECT (
id NUMBER,
name VARCHAR2(20),
addr VARCHAR2(30),
ORDER MEMBER FUNCTION match (c Customer) RETURN INTEGER
);
CREATE TYPE BODY Customer AS
ORDER MEMBER FUNCTION match (c Customer) RETURN INTEGER IS
BEGIN
IF id < c.id THEN
RETURN -1; -- any negative number will do
ELSIF id > c.id THEN
RETURN 1; -- any positive number will do
ELSE
RETURN 0;
END IF;
END;
END;
Every order method takes merely two parameters: the built-in parameter SELF & the other object of similar type. If c1 and c2 are Customer objects, a comparison like c1 > c2 calls method match automatically. The method returns a , zero, negative number or a positive number suggesting that the SELF is correspondingly less than, equal to, or greater than the other parameter. If whichever parameter passed to an order method is null, then the method returns a null.