Using Operator DEREF:
You cannot navigate through refs within the PL/SQL procedural statements. Rather than, you should use the operator DEREF in the SQL statement. The DEREF take as its argument that reference to an object, and then returns the value of that object. If the ref is dangling, the DEREF returns a null object.
In the illustration below, you dereference the ref to a Person object. Note that you select the ref from dummy table dual. You do not require specifying an object table and search criteria as each object stored in an object table has an immutable, exclusive object identifier that is a part of every ref to that object.
DECLARE
p1 Person;
p_ref REF Person;
name VARCHAR2(15);
BEGIN
...
/* Suppose that p_ref holds a valid reference
to an object stored in an object table. */
SELECT DEREF(p_ref) INTO p1 FROM dual;
name := p1.last_name;
You can use DEREF in successive SQL statements to dereference refs, as the
following example shows:
CREATE TYPE PersonRef AS OBJECT (p_ref REF Person)
/
DECLARE
name VARCHAR2(15);
pr_ref REF PersonRef;
pr PersonRef;
p Person;
BEGIN
...
/* Assume pr_ref holds a valid reference. */
SELECT DEREF(pr_ref) INTO pr FROM dual;
SELECT DEREF(pr.p_ref) INTO p FROM dual;
name := p.last_name;
...
END
/
The later illustration shows that you cannot use the operator DEREF within procedural statements:
BEGIN
...
p1 := DEREF(p_ref); -- illegal
Within the SQL statements, you can use the dot notation to navigate throughout the object columns to ref attributes and through one ref attribute to the other. You can also navigate through the ref columns to attributes if you use a table alias. For illustration, the syntax below is valid:
table_alias.object_column.ref_attribute
table_alias.object_column.ref_attribute.attribute
table_alias.ref_column.attribute
Suppose that you have to run the SQL*Plus script below that creates object types Address and Person and object table persons:
CREATE TYPE Address AS OBJECT (
street VARCHAR2(35),
city VARCHAR2(15),
state CHAR(2),
zip_code INTEGER)
/
CREATE TYPE Person AS OBJECT (
first_name VARCHAR2(15),
last_name VARCHAR2(15),
birthday DATE,
home_address REF Address, -- shared with other Person objects
phone_number VARCHAR2(15))
/
CREATE TABLE persons OF Person
/
The Ref attribute home_address corresponds to a column in the object table persons that holds refs to the Address objects stored in some another table. After populating the tables, you can select a particular address by de-referencing its ref, as shown:
DECLARE
addr1 Address,
addr2 Address,
...
BEGIN
SELECT DEREF(home_address) INTO addr1 FROM persons p
WHERE p.last_name = 'Derringer';
In the illustration below, you navigate through ref column home_address to attribute the street. In this situation, the table alias is needed.
DECLARE
my_street VARCHAR2(25),
...
BEGIN
SELECT p.home_address.street INTO my_street FROM persons p
WHERE p.last_name = 'Lucas';