Thursday, June 4, 2009

Relating Multiple Tables

Each table contains data that describes exactly one entity. For example, the EMPLOYEES table contains information about employees. Categories of data are listed across the top of each table, and individual cases are listed below. Using a table format, you can readily visualize, understand, and use information. Because data about different entities is stored in different tables, you may need to combine two or more tables to answer a particular question. For example, you may want to know the location of the department where an employee works. In this scenario, you need information from the EMPLOYEES table (which contains data about employees) and the DEPARTMENTS table (which contains information about departments). With an RDBMS you can relate the data in one table to the data in another by using the foreign keys. A foreign key is a column or a set of columns that refer to a primary key in the same table or another table.

You can use the ability to relate data in one table to data in another to organize information in separate, manageable units. Employee data can be kept logically distinct from department data by storing it in a separate table.

Guidelines for Primary Keys and Foreign Keys

• You cannot use duplicate values in a primary key.

• Primary keys generally cannot be changed.

• Foreign keys are based on data values and are purely logical, not physical, pointers.

• A foreign key value must match an existing primary key value or unique key value, or else be null.

• A foreign key must reference either a primary key or unique key column.

What is RDBMS?

In recent years, database management systems (DBMS) have established themselves as the primary means of data storage for information systems ranging from large commercial transaction processing applications to PC-based desktop applications. At the heart of most of today's information systems is a relational database management system (RDBMS). RDBMSs have been the workhorse for data management operations for over a decade and continue to evolve and mature, providing sophisticated storage, retrieval, and distribution functions to enterprise-wide data processing and information management systems. Compared to the file systems, relational database management systems provide organizations with the capability to easily integrate and leverage the massive amounts of operational data into meaningful information systems. The evolution of high-powered database engines such as Oracle7 has fostered the development of advanced "enabling" technologies including client/server, data warehousing, and online analytical processing, all of which comprise the core of today's state-of-the-art information management systems.

Examine the components of the term relational database management system. First, a database is an integrated collection of related data. Given a specific data item, the structure of a database facilitates the access to data related to it, such as a student and all of his registered courses or an employee and his dependents. Next, a relational database is a type of database based in the relational model; non-relational databases commonly use a hierarchical, network, or object-oriented model as their basis. Finally, a relational database management system is the software that manages a relational database. These systems come in several varieties, ranging from single-user desktop systems to full-featured, global, enterprise-wide systems, such as Oracle7.

This blog discusses the basic elements of a relational database management system, the relational database, and the software systems that manage it. Also included is a discussion of nonprocedural data access. If you are a new user to relational database technology, you'll have to change your thinking somewhat when it comes to referencing data nonprocedurally.

The Relational Database Model

Most of the database management systems used by commercial applications today are based on one of three basic models: the hierarchical model, the network model, or the relational model. The following sections describe the various differences and similarities of the models.

Hierarchical and Network Models

The first commercially available database management systems were of the CODASYL type, and many of them are still in use with mainframe-based, COBOL applications. Both network and hierarchical databases are quite complex in that they rely on the use of permanent internal pointers to relate records to each other. For example, in an accounts payable application, a vendor record might contain a physical pointer in its record structure that points to purchase order records. Each purchase order record in turn contains pointers to purchase order line item records.

The process of inserting, updating, and deleting records using these types of databases requires synchronization of the pointers, a task that must be performed by the application. As you might imagine, this pointer maintenance requires a significant amount of application code (usually written in COBOL) that at times can be quite cumbersome.

Elements of the Relational Model

Relational databases rely on the actual attribute values as opposed to internal pointers to link records. Instead of using an internal pointer from the vendor record to purchase order records, you would link the purchase order record to the vendor record using a common attribute from each record, such as the vendor identification number.

Although the concepts of academic theory underlying the relational model are somewhat complex, you should be familiar with are some basic concepts and terminology. Essentially, there are three basic components of the relational model: relational data structures, constraints that govern the organization of the data structures, and operations that are performed on the data structures.

Relational Data Structures

The relational model supports a single, "logical" structure called a relation, a two-dimensional data structure commonly called a table in the "physical" database. Attributes represent the atomic data elements that are related by the relation. For example, the Customer relation might contain such attributes about a customer as the customer number, customer name, region, credit status, and so on.


Key Values and Referential Integrity


Attributes are grouped with other attributes based on their dependency on a primary key value. A primary key is an attribute or group of attributes that uniquely identifies a row in a table. A table has only one primary key, and as a rule, every table has one. Because primary key values are used as identifiers, they cannot be null. Using the conventional notation for relations, an attribute is underlined to indicate that it is the primary key of the relation. If a primary key consists of several attributes, each attribute is underlined.

You can have additional attributes in a relation with values that you define as unique to the relation. Unlike primary keys, unique keys can contain null values. In practice, unique keys are used to prevent duplication in the table rather than identify rows. Consider a relation that contains the attribute, United States Social Security Number (SSN). In some rows, this attribute may be null in since not every person has a SSN; however for a row that contains a non-null value for the SSN attribute, the value must be unique to the relation.

Linking one relation to another typically involves an attribute that is common to both relations. The common attributes are usually a primary key from one table and a foreign key from the other. Referential integrity rules dictate that foreign key values in one relation reference the primary key values in another relation. Foreign keys might also reference the primary key of the same relation. Figure illustrates two foreign key relationships.



Oracle and Client/Server


Oracle Corporation's reputation as a database company is firmly established in its full-featured, high-performance RDBMS server. With the database as the cornerstone of its product line, Oracle has evolved into more than just a database company, complementing its RDBMS server with a rich offering of well-integrated products that are designed specifically for distributed processing and client/server applications. As Oracle's database server has evolved to support large-scale enterprise systems for transaction processing and decision support, so too have its other products, to the extent that Oracle can provide a complete solution for client/server application development and deployment. This chapter presents an overview of client/server database systems and the Oracle product architectures that support their implementation.

An Overview of Client/Server Computing

The premise of client/server computing is to distribute the execution of a task among multiple processors in a network. Each processor is dedicated to a specific, focused set of subtasks that it performs best, and the end result is increased overall efficiency and effectiveness of the system as a whole. Splitting the execution of tasks between processors is done through a protocol of service requests; one processor, the client, requests a service from another processor, the server. The most prevalent implementation of client/server processing involves separating the user interface portion of an application from the data access portion.

On the client, or front end, of the typical client/server configuration is a user workstation operating with a Graphical User Interface (GUI) platform, usually Microsoft Windows, Macintosh, or Motif. At the back end of the configuration is a database server, often managed by a UNIX, Netware, Windows NT, or VMS operating system.

Client/server architecture also takes the form of a server-to-server configuration. In this arrangement, one server plays the role of a client, requesting database services from another server. Multiple database servers can look like a single logical database, providing transparent access to data that is spread around the network.

Designing an efficient client/server application is somewhat of a balancing act, the goal of which is to evenly distribute execution of tasks among processors while making optimal use of available resources. Given the increased complexity and processing power required to manage a graphical user interface (GUI) and the increased demands for throughput on database servers and networks, achieving the proper distribution of tasks is challenging. Client/server systems are inherently more difficult to develop and manage than traditional host-based application systems because of the following challenges:

The components of a client/server system are distributed across more varied types of processors. There are many more software components that manage client, network, and server functions, as well as an array of infrastructure layers, all of which must be in place and configured to be compatible with each other.

The complexity of GUI applications far outweighs that of their character-based predecessors. GUIs are capable of presenting much more information to the user and providing many additional navigation paths to elements of the interface.

Troubleshooting performance problems and errors is more difficult because of the increased number of components and layers in the system.

Databases in a Client/Server Architecture

Client/server technologies have changed the look and architecture of application systems in two ways. Not only has the supporting hardware architecture undergone substantial changes, but there have also been significant changes in the approach to designing the application logic of the system.

Prior to the advent of client/server technology, most Oracle applications ran on a single node. Typically, a character-based SQL*Forms application would access a database instance on the same machine with the application and the RDBMS competing for the same CPU and memory resources. Not only was the system responsible for supporting all the database processing, but it was also responsible for executing the application logic. In addition, the system was burdened with all the I/O processing for each terminal on the system; each keystroke and display attribute was controlled by the same processor that processed database requests and application logic.

Client/server systems change this architecture considerably by splitting all of the interface management and much of the application processing from the host system processor and distributing it to the client processor.

Combined with the advances in hardware infrastructure, the increased capabilities of RDBMS servers have also contributed to changes in the application architecture. Prior to the release of Oracle7, Oracle's RDBMS was less sophisticated in its capability to support the processing logic necessary to maintain the integrity of data in the database. For example, primary and foreign key checking and enforcement was performed by the application. As a result, the database was highly reliant on application code for enforcement of business rules and integrity, making application code bulkier and more complex. Figure 2.1 illustrates the differences between traditional host-based applications and client/server applications. Client/server database applications can take advantage of the Oracle7 server features for implementation of some of the application logic.