Using operator deref - manipulating objects in pl sql, PL-SQL Programming

Assignment Help:

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';


Related Discussions:- Using operator deref - manipulating objects in pl sql

Package specification, The Package Specification The package specificat...

The Package Specification The package specifications contain the public declarations. The scopes of these declarations are local to your database representation and global to t

Pragma restrict_references in pl sql, Using Pragma RESTRICT_REFERENCES: ...

Using Pragma RESTRICT_REFERENCES: The function called from the SQL statements should obey certain rules meant to control the side effects. To check for violation of the rules,

Ending transactions, Ending Transactions A good quality programming pr...

Ending Transactions A good quality programming practice is to commit or roll back every transaction explicitly. Whether you rollback or issue the commit in your PL/SQL program

Level - sql pseudocolumns, LEVEL You use the LEVEL with the SELECT CON...

LEVEL You use the LEVEL with the SELECT CONNECT BY statement to categorize rows from a database table into a tree structure. The LEVEL returns the level number of a node in a

Fetching with a cursor, Fetching with a Cursor The FETCH statements re...

Fetching with a Cursor The FETCH statements retrieve the rows in the result set one at a time. After each and every fetch, the cursor advance to the next row in the result set

Implicit cursors, Implicit Cursors The Oracle implicitly opens a curso...

Implicit Cursors The Oracle implicitly opens a cursor to process each SQL statement not related with an explicitly declared cursor. The PL/SQL lets you refer to the most recen

Name resolution-naming conventions, Name Resolution In potentially uncer...

Name Resolution In potentially uncertain SQL statements, the names of the database columns take precedence over the names of the local variables and formal parameters. For e.g.

Theory of catastrophism or catalysm - origin of life, THEO R Y OF CATASTR...

THEO R Y OF CATASTROPHISM OR CATALYSM (CUVIER 1769-1832) - The world has passed thorugh several stages and at the end of each stage there was a catastrophe killing all the

Using subqueries, Using Subqueries A subquery is a query (typically ...

Using Subqueries A subquery is a query (typically enclosed by parentheses) that appears within another SQL data manipulation statement. If evaluated, the subquery gives a va

Full time system administrator, Need Windows and Linux system Administrator...

Need Windows and Linux system Administrator We are seeking a part time system administrator to take care of our servers. Your things to do would add, but not limited to: -

Write Your Message!

Captcha
Free Assignment Quote

Assured A++ Grade

Get guaranteed satisfaction & time on delivery in every assignment order you paid with us! We ensure premium quality solution document along with free turntin report!

All rights reserved! Copyrights ©2019-2020 ExpertsMind IT Educational Pvt Ltd