CREATE TABLE pets OF Pet_t (PRIMARY KEY (tag_no)) NESTED TABLE vaccinations STORE AS pet_vaccinations_tab; Using this separate persons table and the REF attribute will allow the existence of people outside the context of their pets (something the pet-obsessed may not envision, but probably a good idea from a design point of view). In this context, REF is called a type modifier. Does a REF sound a lot like a foreign key? While there are important differences between REFs and foreign keys (see Table 18.2), Oracle actually claims that REFs, are "more reliable and persistent" than foreign keys -- probably because REFs do not refer to user-changeable values, but rather to invisible internal values. In fact, the problem with REFs is that they are too persistent. Oracle currently allows you to delete an object that is the target of a REF without deleting the reference to it. They even dignify this state with a name: a dangling REF. This is roughly equivalent to what would happen if you delete a department record without changing the records of employees in that department. There is no declarative way to prevent dangling REFs, but it should not be too challenging to do so by implementing pre-delete triggers on the table that contains the "parent" objects.[ 11] To make life somewhat easier, Oracle provides a predicate, IS DANGLING, to test for this condition: [11] It is also possible to use a foreign key in combination with a REF. To do so, you would include an attribute for the foreign key in the Pet_t specification and include a FOREIGN KEY clause in the CREATE TABLE statement. UPDATE pets SET owner_ref = NULL WHERE owner_ref IS DANGLING; Table 18.2: Chief Differences between Foreign Keys and REFs Characteristic Foreign Key REF Who defines the value used as the "pointer?" User (programmer) System Requirements on the parent Must have primary or unique key Must be an object table or object view Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. Only allows insertions of child if parent exists (or if the referencing columns are null)? Yes, when enabled Yes, since you can only insert a "real" REF Can be defined in such a way that the child may be associated with one of several possible parents? No (although foreign keys have a little-known ability to point to all of several possible parents) Yes; by default, a REF can refer to any row object of the given type Can declaratively restrict the scope of the child so that it can point to only one given parent table? No Yes (by using the SCOPE clause in the CREATE TABLE command) Restricts updates of the parent key when children exist? Yes Yes; object identifiers are not updateable Can prevent the deletion of parent if children exist? Yes No Can cascade deletions of the parent to child (objects)? Yes, with ON DELETE CASCADE No Default type of relationship between parent and child when joined via SQL Equi-join Outer join (when using dot navigation) Parent and child can be on different databases? No; must be enforced with table- level triggers Not in Oracle 8.0.3 NOTE: In Table 18.2, we use the terminology "parent" and "child" only for convenience; these terms are not always accurate descriptions of objects linked via REFs. Oracle has a special syntax for retrieving and modifying data in both SQL and PL/SQL using the REF operator; they also provide a DEREF operator (can you guess why?). We'll look at those operators a bit later. Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. Previous: 18.1 Introduction to Oracle8 Objects Oracle PL/SQL Programming, 2nd Edition Next: 18.3 Syntax for Creating Object Types 18.1 Introduction to Oracle8 Objects Book Index 18.3 Syntax for Creating Object Types The Oracle Library Navigation Copyright (c) 2000 O'Reilly & Associates. All rights reserved. Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. Previous: 18.2 Oracle Objects Example Chapter 18 Object Types Next: 18.4 Manipulating Objects in PL/SQL and SQL 18.3 Syntax for Creating Object Types This section explains the syntax for CREATE TYPE, CREATE TYPE BODY, and some of the other statements you will use when working with Oracle objects. 18.3.1 About Object Types A given object type can have all of the following: ● One default constructor method ● Zero or one comparison methods ● Any number of member methods The default constructor, supplied automatically when you create an object type, allows you to create an object of the corresponding type. You have no direct control over this function (aside from how you have defined the attributes of the object type). The constructor is the only type of method that does not operate on an existing object. Comparison methods are either MAP or ORDER methods (see Section 18.3.6, "Comparing Objects" later in this chapter). They allow you to establish rules so that SQL statements and PL/SQL programs can order, group, and otherwise compare object instances. Comparison methods are always functions. Member methods are either member functions or member procedures. These are where programmers define the bulk of the object's behavior. 18.3.2 CREATE TYPE and DROP TYPE: Creating and Dropping Types The CREATE TYPE statement has the following general format: CREATE [ OR REPLACE ] TYPE <type name> AS OBJECT <attribute name> datatype, ., MEMBER PROCEDURE | FUNCTION <procedure or function Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. spec>, ., [ MAP | ORDER MEMBER FUNCTION <comparison function spec>, . ] [ PRAGMA RESTRICT_REFERENCES (<what to restrict>, restrictions) ] ); As you would expect, you can drop a type using a DROP statement as follows: DROP TYPE <type name> [ FORCE ] ; Parameters have the following meanings: OR REPLACE Tells Oracle that you want to rebuild the type if it should happen to already exist. This will preserve grants. (See "Schema Evolution" later in the chapter for information about the effect this option has on the object type's metadata.) type name A legal Oracle identifier that isn't already in use by any other Oracle database object such as another type, table, or package. May be expressed in "schema dot" notation (e.g., SCOTT. foo). attribute name A legal PL/SQL identifier for the attribute. datatype Any legal Oracle datatype except LONG, LONG RAW, NCHAR, NCLOB, NVARCHAR2, ROWID, BINARY_INTEGER, BOOLEAN, PLS_INTEGER, RECORD, REF CURSOR, % TYPE, %ROWTYPE, or types that exist only within packages. comparison function Defines a function that allows comparison of object values. what to restrict This is either the name of the function or procedure, or the keyword DEFAULT. Using DEFAULT tells Oracle that all member functions and procedures in the object type will have the designated restrictions, without having to list each one in its own RESTRICT_REFERENCES pragma. restrictions One or more of the following: RNDS, WNDS, RNPS, and WNPS (see Chapter 17). FORCE++ Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. Tells Oracle that you want to drop a type even if there are other objects with dependencies on it. Even if you use FORCE, you can only drop a type if it has not been implemented in a table; you must first drop the table(s) before dropping the type. Notice that the syntax for creating the specification is merely a comma-separated list of attributes and methods. There are no semicolons as you would find in a package specification. You cannot impose NOT NULL or DEFAULT constraints at the attribute level. These constraints can, however, be applied to scalar attributes if you create an object table based on type. The syntax is: CREATE TABLE <table name> OF <object type name> (<column constraint>, . ); For example: CREATE TABLE foos OF Foo_t (bar NOT NULL); or, if you wish to name a constraint: CREATE TABLE foos OF Foo_t (CONSTRAINT bar_not_null CHECK (bar IS NOT NULL)); 18.3.3 CREATE TYPE BODY: Creating a Body The syntax for the CREATE TYPE BODY statement is the following: CREATE [ OR REPLACE ] TYPE BODY <type name> AS | IS ( MEMBER PROCEDURE | FUNCTION <procedure or function body>, ., [ MAP | ORDER MEMBER FUNCTION <comparison function body> ] END; Strictly speaking, type bodies are optional; you need a body only if you have created any methods in the specification. Similar to the rules for package specifications and bodies, the methods declared in the specification must match one for one the methods implemented in the body. Methods can be overloaded (see Chapter 15), and the standard rules about overloading apply. 18.3.4 Dot Notation Even if you don't use the object extensions to Oracle, dot notation can be confusing. In SQL, for example, you may have references such as basil.meals.calories, referring to a column called calories in a meals table owned by basil. Add in remote database references, and you might get something like Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. basil.meals.calories@mktg.ny.acme.com. In PL/SQL Version 2 and up, dots are found in record datatypes, table datatype operators, packaged procedure or function references, and elsewhere. In the objects option, there are at least two new opportunities to get confused with dots: object data structures and object methods. (And the discussion below ignores the fact that object names can be preceded by the schema name, as in schema_name.object_name.) 18.3.4.1 Dots in data structures In a PL/SQL program, you can refer to object attributes with dot notation, as in object_name. attribute_name. For example, after declaring and initializing an object my_pet of type Pet_t, we can do this: IF my_pet.sex = 'M' THEN . This variable means "the sex attribute of the object instance my_pet." Referring to nested objects in PL/SQL using dot notation is almost intuitive, as long as you're using embedded objects (that is, the attribute is an object itself, not a REF to an object). CREATE OBJECT Pet_t ( . owner Person_t, -- embedded object, not a REF .); DECLARE the_dalmatian Pet_t; BEGIN . IF the_dalmatian.owner.first_name = 'Persephone' THEN . The IF test above simply checks whether the first name of the owner of the Dalmatian is Persephone. In SQL statements, you can also use dots to navigate the components of nested objects. Even when you have nested objects with REFs, SQL graciously allows you to navigate to the referenced object without actually doing a join: CREATE OBJECT Pet_t ( . owner_ref REF Person_t, .); CREATE TABLE pets of Pet_t; SELECT name, p.owner_ref.first_name Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. FROM pets p; That's a pretty neat trick. No ugly join clause, just an intuitive "do the right thing" call. It works for attributes and member functions that are defined with the appropriate RESTRICT_REFERENCES pragma. But what do we do in PL/SQL? Is this legal? DECLARE the_dalmatian Pet_t; BEGIN SELECT VALUE(p) INTO the_dalmatian FROM pets p WHERE name = 'Cerberus'; . IF the_dalmatian.owner_ref.first_name = 'Persephone' -- invalid THEN . It won't work! In Oracle 8.0.3, you cannot navigate the database through PL/SQL REF variables. Repeat this to yourself like a mantra. Dot notation doesn't help us in this case. For now, you can instead use DEREF, described in detail later on; a future version of Oracle will likely include a built- in package called UTL_REF that supports navigation in PL/SQL. 18.3.4.2 Dots in method invocations When you invoke an object's member function or procedure, the dot syntax is straightforward, as in the following: object_instance_name.function_name (args) object_instance_name.procedure_name (args) If you want to use the output from one method as the input to another, you don't have to use a temporary variable. You can actually chain methods together with dots, as long as they are type compatible: object_name.function_name(args).function_name(args). procedure_name(args) Before we can take a look at an example that chains our Pet_t methods, we'll want to change the specification of print_me. Instead of using the default IN OUT mode of the SELF parameter in a member procedure, we are going to make it an IN. That is, instead of: MEMBER PROCEDURE print_me we want to use: MEMBER PROCEDURE print_me (SELF IN Pet_t) Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. (Remember that we have to make this change in both the object type specification and the object type body.) Why did we make the change? The default IN OUT mode can only accept a SELF parameter that is writeable, and function return values are never writeable. But as an IN-only parameter, SELF can now accept a Pet_t object that is returned from one of the other functions. DECLARE the_pet Pet_t := Pet_t(1949,'Godzilla','BIG MONKEY','M', NULL,NULL,NULL); BEGIN the_pet.set_tag_no(1948).set_photo('gz105.jpg'). print_me(); END; This means "change the tag number of the pet variable to 1948, change its photo to gz105.jpg, and print the result." If you give a little thought to the implications of this convenience feature, you'll realize that it could be valuable to define member functions which return the base object type, so that you can chain them together later. Here are some rules about chaining: ● Methods are invoked in order from left to right. ● The return value of a chained method must be of the object type expected by the method to its right. ● A chained call can include at most a single procedure. ● If your chained call includes a procedure, it must be the right-most method in the chain. ● Be sure that you don't try to use a function's return value (which is read-only) as an IN OUT input to the next method in the chain. 18.3.4.3 Attribute or method? In PL/SQL, there is no automatic visual distinction between an object attribute and an object method unless the method has arguments. That is, in this code fragment: IF my_pet.whatever = 'a value' THEN . we can't immediately determine if "whatever" is an attribute or a method! In some cases, this ambiguity could be a feature, since one day we might want to replace an attribute by a method of the same name. If we want to make our code less mysterious, we can add a trailing empty parameter list to method calls which have no parameters, as in the following: Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. my_pet.print_me(); The empty parentheses notation works for both member functions and member procedures. NOTE: The situation is different in SQL statements. If you call a member function without parameters in a SQL statement, you must use empty parentheses notation. That is, if somefun is a function, don't do this: SELECT p.somefun FROM pets p; -- invalid The statement above fails with an ORA-00904, "invalid column name." The correct syntax is: SELECT p.somefun() FROM pets p; 18.3.5 SELF: The Implied Parameter Because a method can only be called within the context of a particular object instance, it always has an object of the corresponding type as a "parameter." This makes sense because the method will (almost) always need access to that object's attributes. This implied parameter is called SELF. By default, SELF is an IN parameter in member functions, and an IN OUT parameter in member procedures. If we create an object to hold American Kennel Club papers: CREATE TYPE Akc_paper_t AS OBJECT( pet_ref REF Pet_t, issued_on DATE, contents BLOB); the following member function specifications are equivalent: MEMBER FUNCTION print_me RETURN BOOLEAN; MEMBER FUNCTION print_me (SELF Akc_paper_t) RETURN BOOLEAN; MEMBER FUNCTION print_me (SELF IN Akc_paper_t) RETURN BOOLEAN; Similarly, member procedure SELF parameters default to IN OUT, so the following are equivalent to one another: MEMBER PROCEDURE reissue; Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. [...]... database like Oracle could implement an invoker rights model for object methods As with all new technology, we will simply have to wait and see whether such a change Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark comes about, and if it does, what sort of impact it will have on existing applications Previous: 18.2 Oracle Objects Example 18.2 Oracle Objects Example Oracle PL/SQL. .. Manipulating Objects in PL/SQL and SQL 18.4 Manipulating Objects in PL/SQL and SQL The Oracle Library Navigation Copyright (c) 2000 O'Reilly & Associates All rights reserved Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark Previous: 18.3 Syntax for Creating Object Types Chapter 18 Object Types Next: 18.5 Modifying Persistent Objects 18.4 Manipulating Objects in PL/SQL and SQL In... 18.3 Syntax for Creating Object Types Oracle PL/SQL Programming, 2nd Edition Book Index Next: 18.5 Modifying Persistent Objects 18.5 Modifying Persistent Objects The Oracle Library Navigation Copyright (c) 2000 O'Reilly & Associates All rights reserved Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark Previous: 18.4 Manipulating Objects in PL/SQL and SQL Chapter 18 Object Types... how Oracle would deal with statements like these Should it do some sort of "munching" average on the objects' attributes, or what? Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark In fact, Oracle allows us to formulate our own comparison rules for the object types we create By defining a special MAP or ORDER member function when we define an object type, we can tell Oracle. .. acting only Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark on the SELF object and on data exchanged via method arguments Construct PL/SQL "container" packages to manage your persistent object tables (this is similar to what you could do in Oracle7 ), but code these packages to reuse logic that is localized in the object type definition When a PL/SQL application needs to... 'cherry')) Oracle provides constructs such as REF( ) and VALUE( ) so you don't need direct access to these hidden columns In fact, you can get the constructor out of SYS_NC_ROWINFO$ in an Oraclesupported manner as follows: SELECT VALUE(f) FROM foos f; VALUE(F)(ID, BARS) FOO_T(1, BAR_T('apple', 'banana', 'cherry')) 18.4.2.2 REFs Please purchase PDF Split-Merge on www.verypdf.com to... object tables using "dot navigation." In PL/SQL, REFs serve as input/output variables DEREF Reverse pointer lookup for REFs Helpful for retrieving the contents of an object when all you know is its REF 18.4.2.1 Object identifiers (OIDs) Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark Have you ever used an arbitrary number (maybe an Oracle sequence) as a table's primary key?... Object identifiers are a lot like your arbitrary numbers, except that they are assigned by Oracle When you create a table of objects, Oracle adds a hidden field that will hold the object identifier for each object Oracle also automatically creates a unique index on this column When you insert an object into the table, Oracle automatically assigns the object a rather large but hidden object identifier (OID)... FETCH INTO In addition, after an object is initialized, it can be stored in the database, and you can then locate and use that object using several new language constructs: q q q REF VALUE DEREF 18.4.1 The Need to Initialize The designers of the PL/SQL language have established a general convention that uninitialized variables are null.[12] Object variables are no exception; the term for this uninitialized... 3, which are non-null but empty when first declared In PL/SQL8 , uninitialized nested tables and uninitialized VARRAYs are, in fact, null Since all pets need a home, we might want to create an address object type: CREATE TYPE Address_t AS OBJECT( street VARCHAR2(40), city VARCHAR2(20), state VARCHAR2(10), Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark country VARCHAR2(3) . later. Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. Previous: 18.1 Introduction to Oracle8 Objects Oracle PL/SQL Programming,. applications. Previous: 18.2 Oracle Objects Example Oracle PL/SQL Programming, 2nd Edition Next: 18.4 Manipulating Objects in PL/SQL and SQL 18.2 Oracle Objects Example