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

Example of tables within a table - sql, Example of Tables within a Table - ...

Example of Tables within a Table - SQL Example: Obtaining C_ER from COURSE and EXAM_MARK SELECT CourseId, CAST (TABLE (SELECT DISTINCT StudentId, Mark FROM EXAM_MARK AS EM

%rowcount, %ROWCOUNT When its cursor or cursor variable is opened, the...

%ROWCOUNT When its cursor or cursor variable is opened, the %ROWCOUNT is zeroed. Before the first fetch, the %ROWCOUNT yields 0. Afterward, it yields the number of rows fetche

Write sql code that will create the writes table, Question: Consider th...

Question: Consider the following relations (primary keys are underlined): AUTHOR (ANo, aname, address, speciality) PUBLISHER (PNo, pname, Location) BOOK (BNo, Title, ISBN,

Declaring subprograms, Declaring Subprograms   You can declare subprog...

Declaring Subprograms   You can declare subprograms in any PL/SQL subprogram, block, or package. But, you should declare subprograms at the end of the declarative part after a

Explicit cursors, Explicit Cursors The set of rows returned by the que...

Explicit Cursors The set of rows returned by the query can include zero, one, or multiple rows, depending on how many rows meet your search criteria. Whenever a query returns

Using cursor attributes - bulk bind performance improvement, Using Cursor A...

Using Cursor Attributes To process the SQL data manipulation statements, the SQL engine must opens an implicit cursor named SQL. This cursor's attributes (%FOUND, %NOTFOUND, %

Sequential control - pl/sql, Sequential Control Dissimilar to the IF and ...

Sequential Control Dissimilar to the IF and LOOP statements, the GOTO and NULL statements are not important to the PL/SQL programming. The configuration of PL/SQL is such that th

Effects of null for table expression, Effects of NULL for Table Expression ...

Effects of NULL for Table Expression Here's an important distinction between expressions denoting tables and expressions denoting multisets of rows: a table expression cannot

Using operator value in pl sql, Using Operator VALUE: As you may expec...

Using Operator VALUE: As you may expect, the operator VALUE returns the value of an object. The VALUE takes its argument a correlation variable.  For illustration, to return a

Parameter aliasing, Parameter Aliasing   To optimize the subprogram ca...

Parameter Aliasing   To optimize the subprogram call, the PL/SQL compiler can decide between the two techniques of the parameter passing. With the by-value techniques, the v

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