A relational database is a logical representation of data that allows the data to be accessed independently of its physical structure. A relational database organizes data in tables. Figure 20.1 illustrates a sample Employees table that might be used in a personnel system. The table stores the attributes of employees. Tables are composed of rows and columns in which values are stored. This table consists of six rows and five columns. The Number column of each row in this table is the table's primary keya column (or group of columns) in a table that requires a unique value that cannot be duplicated in other rows. This guarantees that a primary key value can be used to uniquely identify a row. A primary key that is composed of two or more columns is known as a composite key. Good examples of primary key columns in other applications are an employee ID number in a payroll system and a part number in an inventory systemvalues in each of these columns are guaranteed to be unique. The rows in Fig. 20.1 are displayed in order by primary key. In this case, the rows are listed in increasing (ascending) order, but they could also be listed in decreasing (descending) order or in no particular order at all. As we will demonstrate in an upcoming example, programs can specify ordering criteria when requesting data from a database.
Figure 20.1. Employees table sample data.
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Each column represents a different data attribute. Rows are normally unique (by primary key) within a table, but some column values may be duplicated between rows. For example, three different rows in the Employees table's Department column contain the number 413, indicating that these employees work in the same department.
Different database users are often interested in different data and different relationships among the data. Most users require only subsets of the rows and columns. To obtain these subsets, programs use SQL to define queries that select subsets of the data from a table. For example, a program might select data from the Employees table to create a query result that shows where each department is located, in increasing order by Department number (Fig. 20.2). SQL queries are discussed in Section 20.4. In Section 20.7, you will learn how to use the IDE's Query Builder to create SQL queries.
Department |
Location |
413 |
New Jersey |
611 |
Orlando |
642 |
Los Angeles |
Relational Database Overview Books Database |
Preface
Index
Introduction to Computers, the Internet and Visual C#
Introduction to the Visual C# 2005 Express Edition IDE
Introduction to C# Applications
Introduction to Classes and Objects
Control Statements: Part 1
Control Statements: Part 2
Methods: A Deeper Look
Arrays
Classes and Objects: A Deeper Look
Object-Oriented Programming: Inheritance
Polymorphism, Interfaces & Operator Overloading
Exception Handling
Graphical User Interface Concepts: Part 1
Graphical User Interface Concepts: Part 2
Multithreading
Strings, Characters and Regular Expressions
Graphics and Multimedia
Files and Streams
Extensible Markup Language (XML)
Database, SQL and ADO.NET
ASP.NET 2.0, Web Forms and Web Controls
Web Services
Networking: Streams-Based Sockets and Datagrams
Searching and Sorting
Data Structures
Generics
Collections
Appendix A. Operator Precedence Chart
Appendix B. Number Systems
Appendix C. Using the Visual Studio 2005 Debugger
Appendix D. ASCII Character Set
Appendix E. Unicode®
Appendix F. Introduction to XHTML: Part 1
Appendix G. Introduction to XHTML: Part 2
Appendix H. HTML/XHTML Special Characters
Appendix I. HTML/XHTML Colors
Appendix J. ATM Case Study Code
Appendix K. UML 2: Additional Diagram Types
Appendix L. Simple Types
Index