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
    • Selecting Data from Multiple Tables
    • Types of Joins
    • Join Considerations
    • Summary
    • Q&A
    • Workshop
  • 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
  • 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

Selecting Data from Multiple Tables

Having the capability to select data from multiple tables is one of SQL's most powerful features. Without this capability, the entire relational database concept would not be feasible. Single-table queries are sometimes quite informative, but in the real world, the most practical queries are those whose data is acquired from multiple tables within the database.

As you witnessed in the hour on normalization, a relational database is broken up into smaller, more manageable tables for simplicity and the sake of overall management ease. As tables are divided into smaller tables, the related tables are created with common columns—primary keys. These keys are used to join related tables to one another.

A join combines two or more tables to retrieve data from multiple tables.

You might ask why you should normalize tables if, in the end, you are only going to rejoin the tables to retrieve the data you want. You rarely select all data from all tables, so it is better to pick and choose according to the needs of each individual query. Although performance may suffer slightly due to a normalized database, overall coding and maintenance are much simpler.

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