Lesson 2 | Understanding related object tables (the REF operator) |
Objective | Describe the SQL enhancements that allow you to query related object tables. |
Understanding related Object Tables in Oracle
Within Oracle, references can be obtained for row objects only.
For every row object that is stored within an object table, Oracle generates and associates an object identifier (OID) for it.
However, objects that are stored within a column do not have such a unique identifier. In the following SlideShow, we create an object and reference it within another object by using the REF
pointer:
Using REF Operator
To query a table that is referenced to an object, we can use standard SQL along with the VALUE
, REF
, and DEREF
operators. Let's examine these operators in detail.
Object Oriented Databases
REF operator
Object Unique Identifier will return a REF (reference) of the object, rather than the object itself.
Like the
VALUE
operator,
REF
takes a
correlation variable[1] as an argument. In this context, a correlation variable is simply a table alias.
More on The REF operator
Typically, a REF
value comprises the object's unique identifier (OID) associated with the object table and the row identifier (ROWID) of the row object. The ROWID is used as a means to provide quick access to the object. A REF
can be used for implicit joins and easy navigation. REF
may sound a lot like a foreign key. However, there are important differences. REF
is more reliable and persistent than a foreign key. This is because REFs
do not refer to user-changeable values. Rather, they refer to invisible, internal values such as OID and ROWID.
Objects implemented through TYPEs
Oracle implements Objects through the use of TYPEs, defined in a similar way to packages.
Unlike packages in which the instance of the package is limited to the current session, an instance of an object type can be stored in the database for later use. The definition of the type contains a comma separated list of attributes and properties, defined in the same way as
- package variables, and
- member functions or procedures.
If a type contains member functions or procedures, the procedural work for these elements is defined in the TYPE BODY.
To see how objects can be used, suppose an object is created to represent a person. In this case, a person is defined by three attributes: first_name, last_name and date_of_birth. Returning the age of the person is also desired, so this is included as a member function, get_age.
CREATE OR REPLACE TYPE t_person AS OBJECT (
first_name VARCHAR2(30),
last_name VARCHAR2(30),
date_of_birth DATE,
MEMBER FUNCTION get_age RETURN NUMBER
);
/
Type created.
The following example illustrates this:
SELECT REF(pc)
FROM pet_care_log_obj_table pc
WHERE to_char(pc.last_update_datetime, 'DD-MMM-YY') =
'12-DEC-99';
Executing this SQL returns the following result set:
REF(PC)
------------------------------------------------------
0000280209DEE8F40AC4B911D39B07000000000000DEE8F3E3C4B9
11D39B07000000000000008003370002
DEREF Operator
Given a reference, DEREF
will return the original object. We will learn more about DEREF
later in this module.
The following example illustrates this:
The link below contains additional information about Database Design.
SELECT DEREF(pc.product_id)
FROM pet_care_log_obj_table pc
WHERE to_char(pc.last_update_datetime, 'DD-MMM-YY') =
'12-DEC-99';
Executing this SQL returns the following result set:
DEREF(PC.PRODUCT_ID)(PRODUCT_ID, PRODUCT_NAME,
PACKAGE_ID, CURRENT_INVENTORY_COUNT, STORE_COST,
SALE
---------------------------------------------------
PRODUCT_TYPE(11, 'Fish', NULL, 150, 0, .99,
'23-OCT-99', 'JANET', 'Y')
VALUE operator
The VALUE
operator returns an object, rather than a list of attributes. It takes a correlation variable as an argument. In this context, a correlation variable is simply a table alias. The following example illustrates how the VALUE
operator is used:
SELECT VALUE(pc)
FROM pet_care_log_obj_table pc
WHERE to_char(pc.last_update_datetime, 'DD-MMM-YY') = '12-DEC-99';
The result set of this query is a set of objects, not a set of attributes, as shown below:
VALUE(PC)(PRODUCT_ID, LOG_DATETIME, CREATED_BY_USER,
LOG_TEXT, LAST_UPDATE_DATETIME)
----------------------------------------------------
PET_CARE_LOG_TYPE(0000220208DEE8F3F3C4B911D39B07000
000000000DEE8F3E2C4B911D39B07000000000000, '12-DE
C-99', 'MARK', 'Fish that are sold must be written
down on the FISH LOG sheet for proper funds if
needed.', '12-DEC-99')
[1]
Correlation variable:
A correlation variable is a parameter, which is passed when a function is used, e.g. the VALUE finction requires the table alias to be passed as a parameter or correlation variable.