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Databases: A Beginner s Guide
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stored within each object You might also see variables referred to as instance variables or properties, but I will stick with the term variables for consistency In OO terminology, a method is a piece of application program logic that operates on a particular object and provides a finite function, such as checking a customer s credit limit or updating a customer s address Among the many differences between the OO model and the models already presented, the most significant is that variables can be accessed only through methods This property is called encapsulation The strict definition of object used here applies only to the OO model The general term database object, as used earlier in this chapter, refers to any named item that might be stored in a non-OO database (such as a table, index, or view) As OO concepts have found their way into relational databases, so has the terminology, although often with less precise definitions Figure 1-9 shows the Customer object as an example of OO implementation The circle of methods around the central core of variables reminds us of encapsulation In fact, you can think of an object much like an atom with an electron field of methods and a nucleus of variables Each customer for Northwind would have its own copy of the object structure, called an object instance, much as each individual customer has a copy of the customer record structure in the flat file system
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Add Customer Variables Methods List Customer Update Contact Company ID Company Name Contact Name Address City Country Phone
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Figure 1-9
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The anatomy of an object
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1: Database Fundamentals
At a glance, the OO model looks horribly inefficient because it seems that each instance requires that the methods and the definition of the variables be redundantly stored However, this is not at all the case Objects are organized into a class hierarchy so that the common methods and variable definitions need only be defined once and then inherited by other members of the same class Variables also belong to classes, and thus new data types can be easily incorporated by simply defining a new class for them The OO model also supports complex objects, which are objects composed of one or more other objects Usually, this is implemented using an object reference, where one object contains the identifier of one or more other objects For example, a Customer object might contain a list of Order objects that the customer has placed, and each Order object might contain the identifier of the customer who placed the order The unique identifier for an object is called the object identifier (OID), the value of which is automatically assigned to each object as it is created and is then invariant (that is, the value never changes) The combination of complex objects and the class hierarchy makes OO databases well suited for managing nonscalar data such as drawings and diagrams OO concepts have such benefit that they have found their way into nearly every aspect of modern computer systems For example, the Microsoft Windows Registry (the directory that stores settings and options for some Windows operating systems) has a class hierarchy, and most computer-aided design (CAD) applications use an OO database to store their data
The Object-Relational Model
Although the OO model provides some significant benefits in encapsulating data to minimize the effects of system modifications, the lack of ad hoc query capability has relegated it to a niche market in which complex data is required, but ad hoc query ability is not However, some vendors of relational databases noted the significant benefits of the OO model, particularly its ability to easily map complex data types, and added object-like capability to their relational DBMS products with the hopes of capitalizing on the best of both models Although object purists have never embraced this approach, the tactic appears to have worked to a large degree, with pure OO databases gaining ground only in niche markets The original name given to this type of database was universal database, and although the marketing folks loved the term, it never caught on in technical circles, so the preferred name for the model became object-relational (OR) Through evolution, the Oracle, DB2, and Informix databases can all be said to be OR DBMSs to varying degrees To understand the OR model fully, you need a more detailed knowledge of the relational and OO models However, keep in mind that the OR DBMS provides a blend of desirable features from the object world, such as the storage of complex data types, with the relative simplicity and ease-of-use of the relational model Most industry experts believe that object-relational technology will continue to gain market share
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