Sams Teach Yourself SQL in 24 Hours

By Ron Plew and Ryan Stephens

• Table of Contents
  • Copyright
  • About the Authors
  • Acknowledgments
  • Tell Us What You Think!
  • Introduction
  • Part I: A SQL Concepts Overview
  • Hour 1. Welcome to the World of SQL
  • Part II: Building Your Database
  • Hour 2. Defining Data Structures
  • Hour 3. Managing Database Objects
  • Hour 4. The Normalization Process
  • Hour 5. Manipulating Data
  • Hour 6. Managing Database Transactions
  • Part III: Getting Effective Results from Queries
  • Hour 7. Introduction to the Database Query
  • Hour 8. Using Operators to Categorize Data
  • Hour 9. Summarizing Data Results from a Query
  • Hour 10. Sorting and Grouping Data
  • Hour 11. Restructuring the Appearance of Data
  • Hour 12. Understanding Dates and Times
  • Part IV: Building Sophisticated Database Queries
  • Hour 13. Joining Tables in Queries
  • Hour 14. Using Subqueries to Define Unknown Data
  • Hour 15. Combining Multiple Queries into One
  • Part V: SQL Performance Tuning
  • Hour 16. Using Indexes to Improve Performance
    • What Is an Index?
    • How Do Indexes Work?
    • The CREATE INDEX Command
    • Types of Indexes
    • When Should Indexes Be Considered?
    • When Should Indexes Be Avoided?
    • Summary
    • Q&A
    • Workshop
  • Hour 17. Improving Database Performance
  • Part VI: Using SQL to Manage Users and Security
  • Hour 18. Managing Database Users
  • Hour 19. Managing Database Security
  • Part VII: Summarized Data Structures
  • Hour 20. Creating and Using Views and Synonyms
  • Hour 21. Working with the System Catalog
  • Part VIII: Applying SQL Fundamentals in Today's World
  • Hour 22. Advanced SQL Topics
  • Hour 23. Extending SQL to the Enterprise, the Internet, and the Intranet
  • Hour 24. Extensions to Standard SQL
  • Part IX: Appendixes
  • Appendix A. Common SQL Commands
  • Appendix B. Using MySQL for Exercises
  • Appendix C. Answers to Quizzes and Exercises
  • Appendix D. CREATE TABLE Statements for Book Examples
  • Appendix E. INSERT Statements for Data in Book Examples
  • Appendix F. Glossary
  • Appendix G. Bonus Exercises

What Is an Index?

Simply put, an index is a pointer to data in a table. An index in a database is very similar to an index in the back of a book. For example, if you want to reference all pages in a book that discuss a certain topic, you first refer to the index, which lists all topics alphabetically, and are then referred to one or more specific page numbers. An index in a database works the same way in that a query is pointed to the exact physical location of data in a table. You are actually being directed to the data's location in an underlying file of the database, but as far as you are concerned, you are referring to a table.

Which would be faster, looking through a book page by page for some information or searching the book's index and getting a page number? Of course, using the book's index is the most efficient method. A lot of time can be saved if that book is large. Say you have a small book of just a few pages. In this case, it may be faster to check the pages for the information than to flip back and forth between the index and pages of the book. When a database does not use an index, it is performing what is typically called a full table scan, the same as flipping through a book page by page. Full table scans are discussed in Hour 17, "Improving Database Performance."

An index is stored separately from the table for which the index was created. An index's main purpose is to improve the performance of data retrieval. Indexes can be created or dropped with no effect on the data. However, once an index is dropped, performance of data retrieval may be slowed. An index does take up physical space and often grows larger than the table itself.

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