2 Basic Components of Oracle Objects

Null Objects and Attributes

A table column, object attribute, collection, or collection element is NULL if it has been initialized to NULL or has not been initialized at all. Usually, a NULL value is replaced by an actual value later on.

An object whose value is NULL is called atomically null. An atomically null object is different from one that simply happens to have null values for all its attributes. When all the attributes of an object are null, these attributes can still be changed, and the object's methods can be called. With an atomically null object, you can do neither of these things. In Example 2-1, consider the contacts table which contains the person_typ object type.

CREATE TYPE person_typ AS OBJECT (
  idno           NUMBER,
  name           VARCHAR2(30),
  phone          VARCHAR2(20),
  MAP MEMBER FUNCTION get_idno RETURN NUMBER, 
  MEMBER PROCEDURE display_details ( SELF IN OUT NOCOPY person_typ ) );
/

CREATE TYPE BODY person_typ AS
  MAP MEMBER FUNCTION get_idno RETURN NUMBER IS
  BEGIN
    RETURN idno;
  END;
  MEMBER PROCEDURE display_details ( SELF IN OUT NOCOPY person_typ ) IS
  BEGIN
    -- use the PUT_LINE procedure of the DBMS_OUTPUT package to display details
    DBMS_OUTPUT.PUT_LINE(TO_CHAR(idno) || ' - '  || name || ' - '  || phone);
  END;
END;
/
CREATE TABLE contacts (
  contact        person_typ,
  contact_date   DATE );

INSERT INTO contacts VALUES (
  person_typ (NULL, NULL, NULL), '24 Jun 2003' );

INSERT INTO contacts VALUES (
  NULL, '24 Jun 2003' );

The two INSERT statements give two different results. In both cases, Oracle allocates space in contacts for a new row and sets its DATE column to the value given. But in the first case, Oracle allocates space for an object in the contact column and sets each of the object's attributes to NULL. In the second case, Oracle sets the person_typ field itself to NULL and does not allocate space for an object.

In some cases, you can omit checks for null values. A table row or row object cannot be null. A nested table of objects cannot contain an element whose value is NULL.

A nested table or array can be null, so you do need to handle that condition. A null collection is different from an empty one, that is, a collection containing no elements.

Character Length Semantics

Lengths for character types CHAR and VARCHAR2 may be specified as a number of characters, instead of bytes, in object attributes and collections even if some of the characters consist of multiple bytes.

To specify character-denominated lengths for CHAR and VARCHAR2 attributes, you add a qualifier char to the length specification.

Like CHAR and VARCHAR2, NCHAR and NVARCHAR2 may also be used as attribute types in objects and collections. These types are always implicitly measured in terms of characters, so no char qualifier is used.

For example, the following statement creates an object with both a character-length VARCHAR2 attribute and an NCHAR attribute:

CREATE TYPE employee_typ AS OBJECT ( 
  name        VARCHAR2(30 char), 
  language    NCHAR(10), 
  phone       VARCHAR2(20) );
/

For CHAR and VARCHAR2 attributes whose length is specified without a char qualifier, the default unit of measure characters or bytes is determined by whether the NLS_LENGTH_SEMANTICS initialization parameter is set to CHAR or BYTE.

Constraints for Object Tables

You can define constraints on an object table just as you can on other tables. You can define constraints on the leaf-level scalar attributes of a column object, with the exception of REFs that are not scoped.

Example 2-3 and Example 2-4 illustrate the possibilities. Example 2-3 places a PRIMARY KEY constraint on the idno column of the object table person_extent.

CREATE TYPE location_typ AS OBJECT (
  building_no  NUMBER,
  city         VARCHAR2(40) );
/

CREATE TYPE office_typ AS OBJECT (
  office_id    VARCHAR(10),
  office_loc   location_typ,
  occupant     person_typ );/

CREATE TABLE office_tab OF office_typ (
             office_id      PRIMARY KEY );

The department_mgrs table in Example 2-4 has a column whose type is the object type location_typ defined in Example 2-3. The example defines constraints on scalar attributes of the location_typ objects that appear in the dept_loc column of the table.

CREATE TABLE department_mgrs (
  dept_no     NUMBER PRIMARY KEY, 
  dept_name   CHAR(20),
  dept_mgr    person_typ,
  dept_loc    location_typ,
  CONSTRAINT  dept_loc_cons1
      UNIQUE (dept_loc.building_no, dept_loc.city),
  CONSTRAINT  dept_loc_cons2
       CHECK (dept_loc.city IS NOT NULL) );

INSERT INTO department_mgrs VALUES 
          ( 101, 'Physical Sciences', 
           person_typ(65,'Vrinda Mills', '1-800-555-4412'),
           location_typ(300, 'Palo Alto'));

Indexes for Object Tables

You can define indexes on an object table or on the storage table for a nested table column or attribute just as you can on other tables. For an example of an index on a nested table, see Example 3-4.

You can define indexes on leaf-level scalar attributes of column objects, as shown in Example 2-5. You can only define indexes on REF attributes or columns if the REF is scoped. Here, dept_addr is a column object, and city is a leaf-level scalar attribute of dept_addr that we want to index.

CREATE TABLE department_loc (
  dept_no     NUMBER PRIMARY KEY, 
  dept_name   CHAR(20),
  dept_addr   location_typ );

CREATE INDEX  i_dept_addr1
          ON  department_loc (dept_addr.city);

INSERT INTO department_loc VALUES
          ( 101, 'Physical Sciences',
           location_typ(300, 'Palo Alto'));
INSERT INTO department_loc VALUES 
          ( 104, 'Life Sciences', 
           location_typ(400, 'Menlo Park'));
INSERT INTO department_loc VALUES 
          ( 103, 'Biological Sciences', 
           location_typ(500, 'Redwood Shores'));

Wherever Oracle expects a column name in an index definition, you can also specify a scalar attribute of an object column.

Triggers for Object Tables

You can define triggers on an object table just as you can on other tables. You cannot define a trigger on the storage table for a nested table column or attribute. You cannot modify LOB values in a trigger body. Otherwise, there are no special restrictions on using object types with triggers.

Example 2-6 defines a trigger on the office_tab table defined in "Constraints for Object Tables".

CREATE TABLE movement (
     idno           NUMBER,
     old_office     location_typ,
     new_office     location_typ );

CREATE TRIGGER trigger1
  BEFORE UPDATE
             OF  office_loc
             ON  office_tab
   FOR EACH ROW
           WHEN  (new.office_loc.city = 'Redwood Shores')
   BEGIN
     IF :new.office_loc.building_no = 600 THEN
      INSERT INTO movement (idno, old_office, new_office)
       VALUES (:old.occupant.idno, :old.office_loc, :new.office_loc);
     END IF;
   END;/

INSERT INTO movement VALUES 
   ( 101, location_typ(300, 'Palo Alto'),
     location_typ(400, 'Menlo Park'));

Rules for REF Columns and Attributes

In Oracle, a REF column or attribute can be unconstrained or constrained using a SCOPE clause or a referential constraint clause. When a REF column is unconstrained, it may store object references to row objects contained in any object table of the corresponding object type.

Oracle does not ensure that the object references stored in such columns point to valid and existing row objects. Therefore, REF columns may contain object references that do not point to any existing row object. Such REF values are referred to as dangling references.

A REF column may be constrained to be scoped to a specific object table. All the REF values stored in a column with a SCOPE constraint point at row objects of the table specified in the SCOPE clause. The REF values may, however, be dangling.

A REF column may be constrained with a REFERENTIAL constraint similar to the specification for foreign keys. The rules for referential constraints apply to such columns. That is, the object reference stored in these columns must point to a valid and existing row object in the specified object table.

PRIMARY KEY constraints cannot be specified for REF columns. However, you can specify NOT NULL constraints for such columns.

Name Resolution

Oracle SQL lets you omit qualifying table names in some relational operations. For example, if dept_addr is a column in the department_loc table and old_office is a column in the movement table, you can use the following:

SELECT * FROM department_loc WHERE EXISTS 
  (SELECT * FROM movement WHERE dept_addr = old_office);

Oracle determines which table each column belongs to.

Using the dot notation, you can qualify the column names with table names or table aliases to make things more maintainable. For example:

SELECT * FROM department_loc WHERE EXISTS 
  (SELECT * FROM movement WHERE department_loc.dept_addr = movement.old_office);

SELECT * FROM department_loc d WHERE EXISTS 
  (SELECT * FROM movement m WHERE d.dept_addr = m.old_office);

In some cases, object-relational features require you to specify the table aliases.

Restriction on Using User-Defined Types with a Remote Database

User-defined types (specifically, types declared with a SQL CREATE TYPE statement, as opposed to types declared within a PL/SQL package) are currently useful only within a single database. You cannot use a database link to do any of the following:

• Connect to a remote database to select, insert, or update a user-defined type or an object REF on a remote table

  You can use the CREATE TYPE statement with the optional keyword OID to create a user-specified object identifier (OID) that allows an object type to be used in multiple databases. See the discussion on assigning an OID to an object type in the Oracle Database Data Cartridge Developer's Guide.

• Use database links within PL/SQL code to declare a local variable of a remote user-defined type

• Convey a user-defined type argument or return value in a PL/SQL remote procedure call.

Member Methods

Member methods are the means by which an application gains access to an object instance's data. You define a member method in the object type for each operation that you want an object of that type to be able to perform. For example, the method get_sum() that sums the total cost of a purchase order's line items operates on the data of a particular purchase order and is a member method.

Member methods have a built-in parameter named SELF that denotes the object instance on which the method is currently being invoked. Member methods can reference the attributes and methods of SELF without a qualifier. This makes it simpler to write member methods. In Example 2-8 the code shows a method declaration that takes advantage of SELF to omit qualification of the attributes hgt, len, and wth.

CREATE TYPE solid_typ AS OBJECT (
  len    INTEGER,
  wth    INTEGER,
  hgt    INTEGER,
  MEMBER FUNCTION surface RETURN INTEGER,
  MEMBER FUNCTION volume RETURN INTEGER,
  MEMBER PROCEDURE display (SELF IN OUT NOCOPY solid_typ) );
/

CREATE TYPE BODY solid_typ AS
  MEMBER FUNCTION volume RETURN INTEGER IS
  BEGIN
    RETURN len * wth * hgt;
 -- RETURN SELF.len * SELF.wth * SELF.hgt; -- equivalent to previous line 
  END;
  MEMBER FUNCTION surface RETURN INTEGER IS
  BEGIN -- not necessary to include SELF prefix in following line
    RETURN 2 * (len * wth + len * hgt + wth * hgt);
  END;
  MEMBER PROCEDURE display (SELF IN OUT NOCOPY solid_typ) IS
  BEGIN
    DBMS_OUTPUT.PUT_LINE('Length: ' || len || ' - '  || 'Width: ' || wth 
                          || ' - '  || 'Height: ' || hgt);
    DBMS_OUTPUT.PUT_LINE('Volume: ' || volume || ' - ' || 'Surface area: ' 
                          || surface);
  END;
END;
/

SELF does not need to be explicitly declared, although it can be. It is always the first parameter passed to the method.

• In member functions, if SELF is not declared, its parameter mode defaults to IN.

• In member procedures, if SELF is not declared, its parameter mode defaults to IN OUT. The default behavior does not include the NOCOPY compiler hint.

You can invoke a member method using the dot notation object_variable.method(). This notation specifies the object on which to invoke the method, then the method to call. Any parameters must be placed inside the required parentheses. See also "Using SELF IN OUT NOCOPY with Member Procedures".

Methods for Comparing Objects

The values of a scalar datatype such as CHAR or REAL have a predefined order, which allows them to be compared. But an object type, such as a person_typ, which can have multiple attributes of various datatypes, has no predefined axis of comparison. To be able to compare and order variables of an object type, you must specify a basis for comparing them. Two special kinds of member methods can be defined for doing this: map methods and order methods.

CREATE TYPE rectangle_typ AS OBJECT ( 
  len NUMBER,
  wid NUMBER,
  MAP MEMBER FUNCTION area RETURN NUMBER);
/

CREATE TYPE BODY rectangle_typ AS 
  MAP MEMBER FUNCTION area RETURN NUMBER IS
  BEGIN
     RETURN len * wid;
  END area;
END;
/

CREATE TYPE location_typ AS OBJECT (
  building_no  NUMBER,
  city         VARCHAR2(40),
  ORDER MEMBER FUNCTION match (l location_typ) RETURN INTEGER );/
CREATE TYPE BODY location_typ AS 
  ORDER MEMBER FUNCTION match (l location_typ) RETURN INTEGER IS 
  BEGIN 
    IF building_no < l.building_no THEN
      RETURN -1;               -- any negative number will do
    ELSIF building_no > l.building_no THEN 
      RETURN 1;                -- any positive number will do
    ELSE 
      RETURN 0;
    END IF;
  END;
END;/

Static Methods

Static methods are invoked on the object type, not its instances. You use a static method for operations that are global to the type and do not need to reference the data of a particular object instance. A static method has no SELF parameter.

You invoke a static method by using the dot notation to qualify the method call with the name of the object type, such as:

type_name.method()

See "Static Methods" for information on design considerations.

Constructor Methods

Every object type has a constructor method implicitly defined for it by the system. A constructor method is a function that returns a new instance of the user-defined type and sets up the values of its attributes. The system implicitly defines a constructor function called the attribute value constructor for all object types that have attributes.

Consider the person_typ object type that is defined in Example 2-1. The name of the constructor method is simply the name of the object type, as shown in the following:

person_typ (1, 'John Smith', '1-800-555-1212'),

A literal invocation of a constructor method is a call to the constructor method in which any arguments are either literals, or further literal invocations of constructor methods. For example:

CREATE TABLE people_tab OF person_typ;

INSERT INTO people_tab VALUES (
       person_typ(101, 'John Smith', '1-800-555-1212') );

You can also define constructor functions of your own called user-defined constructors to create and initialize objects of such types. Attribute value constructors are convenient to use because they already exist, but user-defined constructors have some important advantages with respect to type evolution. See "Advantages of User-Defined Constructors" for information on user-defined constructors and their advantages. See "Constructor Methods for Collections" for information on user-defined constructors for collections.

External Implemented Methods

You can use PL/SQL to invoke external subprograms that have been written in other languages. This provides access to the strengths and capabilities of those languages.

Types and Subtypes

A subtype can be derived from a supertype either directly, or indirectly through intervening levels of other subtypes. A subtype can directly derive only from a single supertype: it cannot derive jointly from more than one. A supertype can have multiple sibling subtypes, but a subtype can have at most one direct parent supertype. In other words, Oracle supports only single inheritance, not multiple inheritance.

A subtype is derived from a supertype by defining a specialized variant of the supertype. For example, from a person_typ object type you might derive the specialized types student_typ and employee_typ. Each of these subtypes is still at bottom a person_typ, but a special kind of person. What makes a subtype special and distinguishes it from its parent supertype is some change made in the subtype to the attributes or methods that the subtype received from its parent.

Figure 2-1 Subtypes

Description of the illustration adobj027.gif

An object type's attributes and methods make the type what it is: they are its essential, defining features. If a person_typ object type has the three attributes idno, name, and phone and the method get_idno(), then any object type that is derived from person_typ will have these same three attributes and a method get_idno(). A subtype is a special case of its parent type, not a totally different kind of thing. As such, it shares with its parent type the features that make the general type what it is.

You can specialize the attributes or methods of a subtype in these ways:

• Add new attributes that its parent supertype does not have.

  For example, you might specialize student_typ as a special kind of person_typ by adding to its definition an attribute for major. A subtype cannot drop or change the type of an attribute it inherited from its parent; it can only add new attributes.

• Add entirely new methods that the parent does not have.

• Change the implementation of some of the methods a subtype inherits from its parent so that the subtype's version executes different code from the parent's.

  For example, a shape object type might define a method calculate_area(). Two subtypes of shape, rectilinear_shape, and circular_shape, might each implement this method in a different way. See "Inheriting, Overloading, and Overriding Methods".

Attributes and methods that a subtype gets from its parent type are said to be inherited. This means more than just that the attributes and methods are patterned on the parent's when the subtype is defined. With object types, the inheritance link remains live. Any changes made later on to the parent type's attributes or methods are also inherited so that the changes are reflected in the subtype as well. Unless a subtype reimplements an inherited method, it always contains the same core set of attributes and methods that are in the parent type, plus any attributes and methods that it adds.

Remember, a child type is not a different type from its parent; it is a particular kind of that type. If the general definition of person_typ ever changes, the definition of student_typ changes also.

The inheritance relationship that holds between a supertype and its subtypes is the source of both much of the power of objects and much of their complexity. It is a very powerful feature to be able to change a method in a supertype and have the change take effect in all the subtypes downstream just by recompiling. But this same capability means that you have to think about such things as whether you want to allow a type to be specialized or a method to be redefined. Similarly, it is a powerful feature for a table or column to be able to contain any type in a hierarchy, but then you must decide whether to allow this in a particular case, and you may need to constrain DML statements and queries so that they pick out from the type hierarchy just the range of types that you want. The following sections address these aspects of working with objects.

FINAL and NOT FINAL Types and Methods

The definition of an object type determines whether subtypes can be derived from that type. To permit subtypes, the object type must be defined as not final. This is done by including the NOT FINAL keyword in its type declaration, as shown in Example 2-11.

CREATE TYPE person_typ AS OBJECT (
   idno           NUMBER,
   name           VARCHAR2(30),
   phone          VARCHAR2(20)) 
NOT FINAL;
/

The preceding statement declares person_typ to be a not final type such that subtypes of person_typ can be defined. By default, an object type is declared as final and subtypes cannot be derived from it.

You can change a final type to a not final type and vice versa with an ALTER TYPE statement. For example, the following statement changes person_typ to a final type:

ALTER TYPE person_typ FINAL;

You can alter a type from NOT FINAL to FINAL only if the target type has no subtypes.

Methods can also be declared to be final or not final. If a method is declared to be final, subtypes cannot override it by providing their own implementation. Unlike types, methods are not final by default and must be explicitly declared to be final.

Example 2-12 creates a not final type containing a final member function.

CREATE TYPE person_typ AS OBJECT (
   idno           NUMBER,
   name           VARCHAR2(30),
   phone          VARCHAR2(20),
   FINAL MAP MEMBER FUNCTION get_idno RETURN NUMBER)
NOT FINAL;
/

See "Redefining Methods".

Creating Subtypes With Overriding Methods

You can create a subtype using a CREATE TYPE statement that specifies the immediate parent of the subtype with an UNDER clause. Example 2-13 shows the creation of the parent or supertype person_typ object; Example 2-14, Example 2-15, and Example 2-16 show the definition of the subtypes.

Note the methods that are created in the supertype body of Example 2-13. In Example 2-14, Example 2-15, and Example 2-16, the show() function of the parent type is overridden to specifications for each subtype. At the same time, the static show_super() function is designed so that it can be called as it is in the supertype from every subtype under the person_typ parent object.

CREATE TYPE person_typ AS OBJECT (
  idno           NUMBER,
  name           VARCHAR2(30),
  phone          VARCHAR2(20),
  MAP MEMBER FUNCTION get_idno RETURN NUMBER,
  STATIC FUNCTION show_super (person_obj in person_typ) RETURN VARCHAR2,
  MEMBER FUNCTION show RETURN VARCHAR2)
  NOT FINAL;
/

CREATE TYPE BODY person_typ AS
  MAP MEMBER FUNCTION get_idno RETURN NUMBER IS
  BEGIN
    RETURN idno;
  END;

-- static function that can be called by subtypes
 STATIC FUNCTION show_super (person_obj in person_typ) RETURN VARCHAR2 IS
  BEGIN
    RETURN 'Id: ' || TO_CHAR(person_obj.idno) || ', Name: ' || person_obj.name;
  END;

-- function that can be overriden by subtypes 
  MEMBER FUNCTION show RETURN VARCHAR2 IS
  BEGIN
    RETURN person_typ.show_super ( SELF );
  END; 

END;
/

Example 2-14 creates student_typ as a subtype of person_typ. As a subtype of person_typ, student_typ inherits all the attributes declared in or inherited by person_typ and any methods inherited by person_typ or declared in person_typ.

The statement that defines student_typ specializes person_typ by adding two new attributes. In addition, the show() function is overridden to display the new attribute major. New attributes declared in a subtype must have names that are different from the names of any attributes or methods declared in any of its supertypes, higher up in its type hierarchy.

CREATE TYPE student_typ UNDER person_typ (
    dept_id NUMBER,
    major VARCHAR2(30),
    OVERRIDING MEMBER FUNCTION show RETURN VARCHAR2)
    NOT FINAL;
/

CREATE TYPE BODY student_typ AS
  OVERRIDING MEMBER FUNCTION show RETURN VARCHAR2 IS
  BEGIN
    RETURN person_typ.show_super ( SELF ) || ' -- Major: ' || major ;
  END;
  
END;
/

A type can have multiple child subtypes, and these can also have subtypes. Example 2-15 creates another subtype employee_typ under person_typ.

CREATE TYPE employee_typ UNDER person_typ (
    emp_id NUMBER, 
    mgr VARCHAR2(30),
    OVERRIDING MEMBER FUNCTION show RETURN VARCHAR2);
/

CREATE TYPE BODY employee_typ AS
  OVERRIDING MEMBER FUNCTION show RETURN VARCHAR2 IS
  BEGIN
    RETURN person_typ.show_super ( SELF ) || ' -- Employee Id: ' 
           || TO_CHAR(emp_id) || ', Manager: ' || mgr ;
  END;
  
END;
/

A subtype can be defined under another subtype. Again, the new subtype inherits all the attributes and methods that its parent type has, both declared and inherited. Example 2-16 defines a new subtype part_time_student_typ under student_typ. The new subtype inherits all the attributes and methods of student_typ and adds another attribute.

CREATE TYPE part_time_student_typ UNDER student_typ (
  number_hours NUMBER,
  OVERRIDING MEMBER FUNCTION show RETURN VARCHAR2);
/

CREATE TYPE BODY part_time_student_typ AS
  OVERRIDING MEMBER FUNCTION show RETURN VARCHAR2 IS
  BEGIN
    RETURN person_typ.show_super ( SELF ) || ' -- Major: ' || major ||
           ', Hours: ' || TO_CHAR(number_hours);
  END;
  
END;
/

You can create a table that contains the supertype and subtypes and populate the table as shown with the person_obj_table in Example 2-17.

CREATE TABLE person_obj_table OF person_typ;

INSERT INTO person_obj_table 
  VALUES (person_typ(12, 'Bob Jones', '111-555-1212'));

INSERT INTO person_obj_table 
  VALUES (student_typ(51, 'Joe Lane', '1-800-555-1312', 12, 'HISTORY'));

INSERT INTO person_obj_table 
  VALUES (employee_typ(55, 'Jane Smith', '1-800-555-7765', 
                       100, 'Jennifer Nelson'));

INSERT INTO person_obj_table  
  VALUES (part_time_student_typ(52, 'Kim Patel', '1-800-555-1232', 14,
         'PHYSICS', 20));

You can call the show() function for the supertype and subtypes in the table with the following:

SELECT p.show() FROM person_obj_table p;

With the table populated as illustrated in Example 2-17, the output is similar to:

Note the overridden show() function displays an output specific to the supertype or subtype while the results of the static show_super() are constant.

NOT INSTANTIABLE Types and Methods

A type can be declared to be NOT INSTANTIABLE. If a type is not instantiable, there is no constructor (default or user-defined) for it, and you cannot instantiate instances of that type (objects, in other words). You might use this option with types that you intend to use solely as supertypes of specialized subtypes that you do instantiate. The following pseudocode provides an example.

A method can also be declared to be not instantiable. Use this option when you want to declare a method in a type without implementing the method there. A type that contains a non-instantiable method must itself be declared not instantiable, as shown in Example 2-18.

CREATE TYPE person_typ AS OBJECT (
  idno           NUMBER,
  name           VARCHAR2(30),
  phone          VARCHAR2(20),
  NOT INSTANTIABLE MEMBER FUNCTION get_idno RETURN NUMBER)
  NOT INSTANTIABLE NOT FINAL;/

A non-instantiable method serves as a placeholder. You might define a non-instantiable method when you expect every subtype to override the method in a different way. In such a case, there is no point in defining the method in the supertype.

If a subtype does not provide an implementation for every inherited non-instantiable method, the subtype itself, like the supertype, must be declared not instantiable. A non-instantiable subtype can be defined under an instantiable supertype.

You can alter an instantiable type to a non-instantiable type and vice versa with an ALTER TYPE statement. In the following example, the ALTER TYPE statement makes person_typ instantiable:

CREATE TYPE person_typ AS OBJECT (
  idno           NUMBER,
  name           VARCHAR2(30),
  phone          VARCHAR2(20))
  NOT INSTANTIABLE NOT FINAL;/
ALTER TYPE person_typ INSTANTIABLE;

You can alter an instantiable type to a non-instantiable type only if the type has no columns, views, tables, or instances that reference that type, either directly, or indirectly through another type or subtype.

You cannot declare a non-instantiable type to be FINAL, which would be pointless anyway.

Inheriting, Overloading, and Overriding Methods

A subtype automatically inherits all methods (both member and static methods) declared in or inherited by its supertype.

A subtype can redefine methods it inherits, and it can also add new methods. It can even add new methods that have the same names as methods it inherits, such that the subtype ends up containing more than one method with the same name.

Giving a type multiple methods with the same name is called method overloading. Redefining an inherited method to customize its behavior in a subtype is either overriding, in the case of member methods, or hiding, in the case of static methods.

See the examples in "Creating Subtypes With Overriding Methods" and Example 7-7.

Dynamic Method Dispatch

As a result of method overriding, a type hierarchy can define multiple implementations of the same method. For example, in a hierarchy of the types ellipse_typ, circle_typ, sphere_typ, each type might define a method calculate_area() differently.

Figure 2-2 Hierarchy of Types

Description of the illustration adobj025.gif

When such a method is invoked, the type of the object instance that invokes it is used to determine which implementation of the method to use. The call is then dispatched to that implementation for execution. This process of selecting a method implementation is called virtual or dynamic method dispatch because it is done at run time, not at compile time.

A method call is dispatched to the nearest implementation, working back up the inheritance hierarchy from the current or specified type. If the call invokes a member method of an object instance, the type of that instance is the current type, and the implementation defined or inherited by that type is used. If the call invokes a static method of a type, the implementation defined or inherited by that specified type is used.

For example, if c1 is an object instance of circle_typ, c1.proc() looks first for an implementation of proc() defined in circle_typ. If none is found, it looks up the supertype chain for an implementation in ellipse_typ. The fact that sphere_typ also defines an implementation is irrelevant because the type hierarchy is searched only upwards, toward the top. Subtypes of the current type are not searched.

Similarly, a call to a static method circle_typ.bar() looks first in circle_typ and then, if necessary, in the supertype(s) of circle_typ. The subtype sphere_typ is not searched.

Substituting Types in a Type Hierarchy

In a type hierarchy, the subtypes are variant kinds of the root, base type. For example, a student_typ type and an employee_typ are kinds of a person_typ. The base type includes these other types.

When you work with types in a type hierarchy, sometimes you want to work at the most general level and, for example, select or update all persons. But sometimes you want to select or update only students, or only persons who are not students.

The (polymorphic) ability to select all persons and get back not only objects whose declared type is person_typ but also objects whose declared subtype is student_typ or employee_typ is called substitutability. A supertype is substitutable if one of its subtypes can substitute or stand in for it in a slot (a variable, column, and so forth) whose declared type is the supertype.

In general, types are substitutable. Object attributes, collection elements and REFs are substitutable. An attribute defined as a REF, type, or collection of type person_typ can hold a REF to, an instance of, or instances of an instance of person_typ or an instance of any subtype of person_typ.

This is what you would expect, given that a subtype is, after all, just a specialized kind of any of its supertypes. Formally, though, a subtype is a type in its own right: it is not the same type as its supertype. A column that holds all persons, including all persons who are students and all persons who are employees, actually holds data of multiple types.

Substitutability comes into play in attributes, columns, and rows (namely, of an object view or object table) declared to be an object type, a REF to an object type, or a collection type.

In principle, object attributes, collection elements and REFs are always substitutable: there is no syntax at the level of the type definition to constrain their substitutability to some subtype. You can, however, turn off or constrain substitutability at the storage level, for specific tables and columns. See "Turning Off Substitutability in a New Table" and "Constraining Substitutability".

Column and Row Substitutability

Object type columns are substitutable, and so are object-type rows in object tables and views. In other words, a column or row defined to be of type t can contain instances of t and any of its subtypes.

For example, consider the person_typ type hierarchy introduced in "Creating Subtypes With Overriding Methods". An object table of person_typ can contain rows of all three types. You insert an instance of a given type using the constructor for that type in the VALUES clause of the INSERT statement as shown in Example 2-17.

Similarly, in a relational table or view, a substitutable column of type person_typ can contain instances of all three types. The following example inserts a person, a student, and a part-time student in the person_typ column contact:

CREATE TABLE contacts (
  contact         person_typ,
  contact_date    DATE );

INSERT INTO contacts 
  VALUES (person_typ (12, 'Bob Jones', '111-555-1212'), '24 Jun 2003' );

INSERT INTO contacts 
  VALUES (student_typ(51, 'Joe Lane', '1-800-555-1312', 12, 'HISTORY'),
         '24 Jun 2003' );

INSERT INTO contacts 
  VALUES (part_time_student_typ(52, 'Kim Patel', '1-800-555-1232', 14,
          'PHYSICS', 20), '24 Jun 2003' );

A newly created subtype can be stored in any substitutable tables and columns of its supertype, including tables and columns that existed before the subtype was created.

Attributes in general can be accessed using the dot notation. Attributes of a subtype of a row or column's declared type can be accessed with the TREAT function. For example:

SELECT TREAT(contact AS student_typ).major FROM contacts;

See "TREAT".

CREATE TABLE person_obj_table OF person_typ;

INSERT INTO person_obj_table
  VALUES (person_typ(20, 'Bob Jones', '111-555-1212'));

SELECT p.object_id, p.object_value FROM person_obj_table p;

CREATE TYPE student_typ UNDER person_typ (
    dept_id   NUMBER,
    major     VARCHAR2(30),
    advisor   person_typ);
/

Creating Subtypes After Creating Substitutable Columns

If you create a subtype, any table that already has substitutable columns of the supertype is automatically enabled to store the new subtype as well. This means that your options for creating subtypes are affected by the existence of such tables. If such a table exists, you can only create subtypes that are substitutable, that is, subtypes that Oracle can enable that table to store.

The following example shows an attempt to create a subtype student_typ under person_typ.

CREATE TYPE person_typ AS OBJECT (
  idno           NUMBER,
  name           VARCHAR2(30),
  phone          VARCHAR2(20))
  NOT FINAL;/

CREATE TYPE employee_typ UNDER person_typ (
    emp_id NUMBER, 
    mgr VARCHAR2(30));
/

CREATE TABLE person_obj_table (p person_typ);

The following statement fails because student_typ has a supertype attribute, and table person_obj_table has a substitutable column p of the supertype.

CREATE TYPE student_typ UNDER person_typ ( -- incorrect CREATE subtype
    advisor person_typ);
/

The following attempt succeeds. This version of the student_typ subtype is substitutable. Oracle automatically enables table person_obj_table to store instances of this new type.

CREATE TYPE student_typ UNDER person_typ (
    dept_id NUMBER,
    major VARCHAR2(30));/
INSERT INTO person_obj_table 
  VALUES (student_typ(51, 'Joe Lane', '1-800-555-1312', 12, 'HISTORY'));

Dropping Subtypes After Creating Substitutable Columns

You can drop a subtype with the VALIDATE option only if no instances of the subtype are stored in any substitutable column of the supertype.

For example, the following statement fails because an instance of student_typ is stored in substitutable column p of table person_obj_table:

DROP TYPE student_typ VALIDATE -- incorrect DROP TYPE ; 

To drop the type, first delete any of its instances in substitutable columns of the supertype:

DELETE FROM person_obj_table WHERE p IS OF (student_typ); 

DROP TYPE student_typ VALIDATE;

Turning Off Substitutability in a New Table

When creating a table, you can turn off all substitutability on a column or attribute, including embedded attributes and collections nested to any level, with the clause NOT SUBSTITUTABLE AT ALL LEVELS.

In the following example, the clause confines column office of a relational table to storing only office_typ instances and disallows any subtype instances:

With object tables, the clause can be applied to the table as a whole, like this:

CREATE TABLE office_tab OF office_typ
  NOT SUBSTITUTABLE AT ALL LEVELS;

Alternatively, the clause can also be applied to turn off substitutability in a particular column that is, for a particular attribute of the object type of the table:

CREATE TABLE office_tab OF office_typ
  COLUMN occupant NOT SUBSTITUTABLE AT ALL LEVELS;

You can specify that the element type of a collection is not substitutable using syntax like the following:

CREATE TABLE people_tab (
    people_column people_typ )
    NESTED TABLE people_column 
      NOT SUBSTITUTABLE AT ALL LEVELS STORE AS people_column_nt;

There is no mechanism to turn off substitutability for REF columns.

Constraining Substitutability

You can impose a constraint that limits the range of subtypes permitted in an object column or attribute to a particular subtype in the declared type's hierarchy. You do this using an IS OF type constraint.

For example, the following statement creates a table of office_typ in which occupants are constrained to just those persons who are employees:

CREATE TABLE office_tab OF office_typ
  COLUMN occupant IS OF (ONLY employee_typ);

Although the type office_typ allows authors to be of type person_typ, the column declaration imposes a constraint to store only instances of employee_typ.

You can only use the IS OF type operator to constrain row and column objects to a single subtype (not several), and you must use the ONLY keyword, as in the preceding example.

You can use either IS OF type or NOT SUBSTITUTABLE AT ALL LEVELS to constrain an object column, but you cannot use both.

Modifying Substitutability

In an existing table, you can change an object column from SUBSTITUTABLE to NOT SUBSTITUTABLE (or from NOT SUBSTITUTABLE to SUBSTITUTABLE) by using an ALTER TABLE statement. To do so, you specify the clause [NOT] SUBSTITUTABLE AT ALL LEVELS for the particular column.

You can modify substitutability only for a specific column; you cannot modify substitutability for an object table as a whole.

The following statement makes column office substitutable:

ALTER TABLE dept_office
  MODIFY COLUMN office SUBSTITUTABLE AT ALL LEVELS;  

The following statement makes the column not substitutable. Notice that it also uses the FORCE keyword. This keyword causes any hidden columns containing typeid information or data for subtype attributes to be dropped:

ALTER TABLE  dept_office
  MODIFY COLUMN office NOT SUBSTITUTABLE AT ALL LEVELS FORCE;

If the FORCE keyword is not used when a column is made not substitutable, the column and all attributes of the type must be FINAL or the ALTER TABLE statement will fail.

A VARRAY column can be modified from SUBSTITUTABLE to NOT SUBSTITUTABLE only if the element type of the varray is final itself and has no embedded types (in its attributes or in their attributes, and so on) that are not final.

See "Hidden Columns for Substitutable Columns and Tables" for more information about hidden columns for typeids and subtype attributes.

Restrictions on Modifying Substitutability

You can change the substitutability of only one column at a time with an ALTER TABLE statement. To change substitutability for multiple columns, you must issue multiple statements.

In an object table, you can modify substitutability for a column only if substitutability was not explicitly set at the table level, for the entire table, when the table was created.

For example, the following attempt to modify substitutability for column address succeeds because substitutability has not been explicitly turned on or off at the table level in the CREATE TABLE statement:

CREATE TABLE office_tab OF office_typ;

ALTER TABLE office_tab
  MODIFY COLUMN occupant NOT SUBSTITUTABLE AT ALL LEVELS FORCE;

However, in the following example, substitutability is explicitly set at the table level, so the attempt to modify the setting for column address fails:

CREATE TABLE office_tab OF office_typ
  NOT SUBSTITUTABLE AT ALL LEVELS;

/* Following SQL statement generates an error: */
ALTER TABLE office_tab 
  MODIFY COLUMN occupant SUBSTITUTABLE AT ALL LEVELS FORCE  -- incorrect ALTER;

A column whose substitutability is already constrained by an IS OF type operator cannot have its substitutability modified with a [NOT] SUBSTITUTABLE AT ALL LEVELS clause. See "Constraining Substitutability" for information about IS OF type.

Assignments Across Types

The assignment rules described in this section apply to INSERT/UPDATE statements, the RETURNING clause, function parameters, and PL/SQL variables.

DECLARE
  var1 person_typ;
  var2 employee_typ;
BEGIN
  var2 := employee_typ(55, 'Jane Smith', '1-800-555-7765', 100, 'Jennifer Nelson');
  var1 := var2;
END;
/

CREATE TYPE person_set AS TABLE OF person_typ;
/

CREATE TYPE student_set AS TABLE OF student_typ;
/

DECLARE
  var1 person_set; 
  var2 student_set;
  elem1 person_typ; 
  elem2 student_typ;
BEGIN
--  var1 := var2;   /* ILLEGAL - collections not of same type */
  var1 := person_set (elem1, elem2);   /* LEGAL : Element is of subtype */
END;
/

Comparisons of Objects, REF Variables, and Collections

This section discusses the comparison operators used in SQL conditions.

CAST

CAST converts one built-in datatype or collection-typed value into another built-in datatype or collection-typed value. For example:

CREATE TYPE person_list_typ AS TABLE OF person_typ;/

SELECT CAST(COLLECT(contact) AS person_list_typ) 
  FROM contacts;

For more information about the SQL CAST function, see Oracle Database SQL Reference.

CURSOR

A CURSOR expression returns a nested cursor. This form of expression is equivalent to the PL/SQL REF CURSOR and can be passed as a REF CURSOR argument to a function.

For more information about the SQL CURSOR expression, see Oracle Database SQL Reference.

DEREF

The DEREF function in a SQL statement returns the object instance corresponding to a REF. The object instance returned by DEREF may be of the declared type of the REF or any of its subtypes.

For example, the following statement returns person_typ objects from the table contact_ref.

SELECT DEREF(c.contact_ref), c.contact_date 
  FROM contacts_ref c;

See "Dereferencing REFs". For more information about the SQL DEREF function, see Oracle Database SQL Reference.

IS OF type

The IS OF type predicate tests object instances for the level of specialization of their type.

For example, the following query retrieves all student instances (including any subtypes of students) stored in the person_obj_table table.

SELECT VALUE(p) 
  FROM person_obj_table p
WHERE VALUE(p) IS OF (student_typ);

For any object that is not of a specified subtype, or a subtype of a specified subtype, IS OF returns FALSE. Subtypes of a specified subtype are just more specialized versions of the specified subtype. If you want to exclude such subtypes, you can use the ONLY keyword. This keyword causes IS OF to return FALSE for all types except the specified types.

In the following example, the statement tests objects in object table person_obj_table, which contains persons, employees, and students, and returns REFs just to objects of the two specified person subtypes employee_typ, student_typ, and their subtypes, if any:

SELECT REF(p) 
  FROM person_obj_table p
WHERE VALUE(p) IS OF (employee_typ, student_typ);

Here is a similar example in PL/SQL. The code does something if the person is an employee or student:

DECLARE 
  var person_typ; 
BEGIN 
  var := employee_typ(55, 'Jane Smith', '1-800-555-7765', 100, 'Jennifer Nelson');
  IF var IS OF (employee_typ, student_typ) THEN 
     DBMS_OUTPUT.PUT_LINE('Var is an employee_typ or student_typ object.');
  ELSE
     DBMS_OUTPUT.PUT_LINE('Var is not an employee_typ or student_typ object.');
  END IF;
END;
/

The following statement returns only students whose most specific or specialized type is student_typ. If the table or view contains any objects of a subtype of student_typ, such as part_time_student_typ, these are excluded. The example uses the TREAT function to convert objects that are students to student_typ from the declared type of the view, person_typ:

SELECT TREAT(VALUE(p) AS student_typ)
  FROM person_obj_table p 
WHERE VALUE(p) IS OF(ONLY student_typ);

To test the type of the object that a REF points to, you can use the DEREF function to dereference the REF before testing with the IS OF type predicate.

For example, if contact_ref is declared to be REF person_typ, you can get just the rows for students as follows:

SELECT * 
  FROM contacts_ref
WHERE DEREF(contact_ref) IS OF (student_typ);

For more information about the SQL IS OF type condition, see Oracle Database SQL Reference.

REF

The REF function in a SQL statement takes as an argument a correlation name for an object table or view and returns a reference (a REF) to an object instance from that table or view. The REF function may return references to objects of the declared type of the table, view, or any of its subtypes. For example, the following statement returns the references to all persons, including references to students and employees, whose idno attribute is 12:

SELECT REF(p) 
  FROM person_obj_table p
  WHERE p.idno = 12;

For more information about the SQL REF function, see Oracle Database SQL Reference.

SYS_TYPEID

The SYS_TYPEID function can be used in a query to return the typeid of the most specific type of the object instance passed to the function as an argument.

The most specific type of an object instance is the type to which the instance belongs that is farthest removed from the root type. For example, if Tim is a part-time student, he is also a student and a person, but his most specific type is part-time student.

The function returns the typeids from the hidden type discriminant column that is associated with every substitutable column. The function returns a null typeid for a final, root type.

The syntax of the function is:

SYS_TYPEID(object_type_value)

Function SYS_TYPEID may be used only with arguments of an object type. Its primary purpose is to make it possible to build an index on a hidden type discriminant column.

All types that do belong to a type hierarchy are assigned a non-null typeid that is unique within the type hierarchy. Types that do not belong to a type hierarchy have a null typeid.

Every type except a final root type belongs to a type hierarchy. A final root type has no types related to it by inheritance:

• It cannot have subtypes derived from it because it is final

• It is not itself derived from some other type because it is a root type, so it does not have any supertypes.

See "Hidden Columns for Substitutable Columns and Tables" for more information about type discriminant columns.

For an example of SYS_TYPEID, consider the substitutable object table person_obj_table, of person_typ. person_typ is the root type of a hierarchy that has student_typ as a subtype and part_time_student_typ as a subtype of student_typ. See Example 2-17.

The following query uses SYS_TYPEID. It gets the name attribute and typeid of the object instances in the person_obj_table table. Each of the instances is of a different type:

SELECT name, SYS_TYPEID(VALUE(p)) typeid FROM person_obj_table p;

See "Hidden Columns for Substitutable Columns and Tables" for information about the type discriminant and other hidden columns. For more information about the SQL SYS TYPEID function, see Oracle Database SQL Reference.

TABLE()

Table functions are functions that produce a collection of rows, a nested table or a varray, that can be queried like a physical database table or assigned to a PL/SQL collection variable. You can use a table function like the name of a database table, in the FROM clause of a query, or like a column name in the SELECT list of a query.

A table function can take a collection of rows as input. An input collection parameter can be either a collection type, such as a VARRAY or a PL/SQL table, or a REF CURSOR.

Use PIPELINED to instruct Oracle to return the results of a table function iteratively. A table function returns a nested table or varray collection type. You query table functions by using the TABLE keyword before the function name in the FROM clause of the query.

For information on TABLE() functions, see Oracle Database Data Cartridge Developer's Guide and Oracle Database PL/SQL User's Guide and Reference.

TREAT

The TREAT function does a runtime check to confirm that an expression can be operated on as if it were of a different specified type in the hierarchy normally, a subtype of the expression s declared type. In other words, the function attempts to treat a supertype instance as a subtype instance to treat a person as a student, for example. Whether this can be done in a given case depends on whether the person in question actually is a student (or student subtype, such as a part-time student). If the person is a student, then the person is returned as a student, with the additional attributes and methods that a student may have. If the person happens not to be a student, TREAT returns NULL in SQL.

The two main uses of TREAT are:

• In narrowing assignments, to modify the type of an expression so that the expression can be assigned to a variable of a more specialized type in the hierarchy: in other words, to set a supertype value into a subtype.

• To access attributes or methods of a subtype of the declared type of a row or column

A substitutable object table or column of type T has a hidden column for every attribute of every subtype of T. These hidden columns are not listed by a DESCRIBE statement, but they contain subtype attribute data. TREAT enables you to access these columns.

The following example shows TREAT used in an assignment where a column of person type is set into a column of employee type. For each row in perscol, TREAT returns an employee type or NULL, depending on whether the given person happens to be an employee.

UPDATE T set empcol = TREAT(perscol AS employee_typ);

In the next example, TREAT returns all (and only) student_typ instances from person_obj_table of type person_typ, a supertype of student_typ. The statement uses TREAT to modify the type of p from person_typ to student_typ.

SELECT TREAT(VALUE(p) AS student_typ)
  FROM person_obj_table p;

For each p, the TREAT modification succeeds only if the most specific or specialized type of the value of p is student_typ or one of its subtypes. If p is a person who is not a student, or if p is NULL, TREAT returns NULL in SQL or, in PL/SQL, raises an exception.

You can also use TREAT to modify the declared type of a REF expression. For example:

SELECT TREAT(REF(p) AS REF student_typ)
  FROM person_obj_table p;

The previous example returns REFs to all student_typ instances. In SQL it returns NULL REFs for all person instances that are not students, and in PL/SQL it raises an exception.

Perhaps the most important use of TREAT is to access attributes or methods of a subtype of a row or column's declared type. The following query retrieves the major attribute of all persons, students and part-time students, who have this attribute. NULL is returned for persons who are not students:

SELECT name, TREAT(VALUE(p) AS student_typ).major major 
  FROM person_obj_table p;

The following query will not work because major is an attribute of student_typ but not of person_typ, the declared type of table persons:

SELECT name, VALUE(p).major major FROM person_obj_table p -- incorrect;

The following is a PL/SQL example:

DECLARE 
  var person_typ; 
BEGIN 
  var := employee_typ(55, 'Jane Smith', '1-800-555-7765', 100, 'Jennifer Nelson');
  DBMS_OUTPUT.PUT_LINE(TREAT(var AS employee_typ).mgr);
END;
/

See "Assignments Across Types" for information on using TREAT in assignments. For more information about the SQL TREAT function, see Oracle Database SQL Reference.

VALUE

In a SQL statement, the VALUE function takes as its argument a correlation variable (table alias) for an object table or object view and returns object instances corresponding to rows of the table or view. The VALUE function may return instances of the declared type of the row or any of its subtypes. InExample 2-34 the query returns all persons, including students and employees, from table person_obj_table of person_typ.

SELECT VALUE(p) FROM person_obj_table p;

To retrieve only part time students, that is, instances whose most specific type is part_time_student_typ, use the ONLY keyword to confine the selection:

SELECT VALUE(p) FROM person_obj_table p 
  WHERE VALUE(p) IS OF (ONLY part_time_student_typ);

In the following example, VALUE is used to update a object instance in an object table:

UPDATE person_obj_table p
   SET VALUE(p) = person_typ(12, 'Bob Jones', '1-800-555-1243')
   WHERE p.idno = 12;

See also Example 3-21, "Using VALUE to Update a Nested Table". For more information about the SQL VALUE function, see Oracle Database SQL Reference.