- Sams Teach Yourself SQL in 24 Hours, Third Edition
- 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
- SQL Definition and History
- SQL Sessions
- Types of SQL Commands
- An Introduction to the Database Used in This Book
- Summary
- Q&A
- Workshop
- Part II: Building Your Database
- Hour 2. Defining Data Structures
- What Is Data?
- Basic Data Types
- Summary
- Q&A
- Workshop
- Hour 3. Managing Database Objects
- What Are Database Objects?
- What Is a Schema?
- A Table: The Primary Storage for Data
- Integrity Constraints
- Summary
- Q&A
- Workshop
- Hour 4. The Normalization Process
- Normalizing a Database
- Summary
- Q&A
- Workshop
- Hour 5. Manipulating Data
- Overview of Data Manipulation
- Populating Tables with New Data
- Updating Existing Data
- Deleting Data from Tables
- Summary
- Q&A
- Workshop
- Hour 6. Managing Database Transactions
- What Is a Transaction?
- What Is Transactional Control?
- Transactional Control and Database Performance
- Summary
- Q&A
- Workshop
- Part III: Getting Effective Results from Queries
- Hour 7. Introduction to the Database Query
- What Is a Query?
- Introduction to the <tt>SELECT</tt> Statement
- Examples of Simple Queries
- Summary
- Q&A
- Workshop
- Hour 8. Using Operators to Categorize Data
- What Is an Operator in SQL?
- Comparison Operators
- Logical Operators
- Conjunctive Operators
- Negating Conditions with the <tt>NOT</tt> Operator
- Arithmetic Operators
- Summary
- Q&A
- Workshop
- Hour 9. Summarizing Data Results from a Query
- What Are Aggregate Functions?
- Summary
- Q&A
- Workshop
- Hour 10. Sorting and Grouping Data
- Why Group Data?
- The <tt>GROUP BY</tt> Clause
- <tt>GROUP BY</tt> Versus <tt>ORDER BY</tt>
- The <tt>HAVING</tt> Clause
- Summary
- Q&A
- Workshop
- Hour 11. Restructuring the Appearance of Data
- The Concepts of ANSI Character Functions
- Various Common Character Functions
- Miscellaneous Character Functions
- Mathematical Functions
- Conversion Functions
- The Concept of Combining Character Functions
- Summary
- Q&A
- Workshop
- Hour 12. Understanding Dates and Times
- How Is a Date Stored?
- Date Functions
- Date Conversions
- Summary
- Q&A
- Workshop
- 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
- What Is a Subquery?
- Embedding a Subquery Within a Subquery
- Summary
- Q&A
- Workshop
- Hour 15. Combining Multiple Queries into One
- Single Queries Versus Compound Queries
- Why Would I Ever Want to Use a Compound Query?
- Compound Query Operators
- Using an <tt>ORDER BY</tt> with a Compound Query
- Using <tt>GROUP BY</tt> with a Compound Query
- Retrieving Accurate Data
- Summary
- Workshop
- Q&A
- Part V: SQL Performance Tuning
- Hour 16. Using Indexes to Improve Performance
- What Is an Index?
- How Do Indexes Work?
- The <tt>CREATE INDEX</tt> Command
- Types of Indexes
- When Should Indexes Be Considered?
- When Should Indexes Be Avoided?
- Summary
- Q&A
- Workshop
- Hour 17. Improving Database Performance
- What Is SQL Statement Tuning?
- Database Tuning Versus SQL Tuning
- Formatting Your SQL Statement
- Full Table Scans
- Other Performance Considerations
- Performance Tools
- Summary
- Q&A
- Workshop
- Part VI: Using SQL to Manage Users and Security
- Hour 18. Managing Database Users
- Users Are the Reason
- The Management Process
- Tools Utilized by Database Users
- Summary
- Q&A
- Workshop
- Hour 19. Managing Database Security
- What Is Database Security?
- How Does Security Differ from User Management?
- What Are Privileges?
- Controlling User Access
- Controlling Privileges Through Roles
- Summary
- Q&A
- Workshop
- Part VII: Summarized Data Structures
- Hour 20. Creating and Using Views and Synonyms
- What Is a View?
- Creating Views
- Dropping a View
- What Is a Synonym?
- Summary
- Q&A
- Workshop
- Hour 21. Working with the System Catalog
- What Is the System Catalog?
- How Is the System Catalog Created?
- What Is Contained in the System Catalog?
- Examples of System Catalog Tables by Implementation
- Querying the System Catalog
- Updating System Catalog Objects
- Summary
- Q&A
- Workshop
- Part VIII: Applying SQL Fundamentals in Today's World
- Hour 22. Advanced SQL Topics
- Advanced Topics
- Cursors
- Stored Procedures and Functions
- Triggers
- Dynamic SQL
- Call-Level Interface
- Using SQL to Generate SQL
- Direct Versus Embedded SQL
- Summary
- Q&A
- Workshop
- Hour 23. Extending SQL to the Enterprise, the Internet, and the Intranet
- SQL and the Enterprise
- Accessing a Remote Database
- Accessing a Remote Database Through a Web Interface
- SQL and the Internet
- SQL and the Intranet
- Summary
- Q&A
- Workshop
- Hour 24. Extensions to Standard SQL
- Various Implementations
- Examples of Extensions from Some Implementations
- Interactive SQL Statements
- Summary
- Q&A
- Workshop
- Part IX: Appendixes
- Appendix A. Common SQL Commands
- SQL Statements
- SQL Clauses
- Appendix B. Using MySQL for Exercises
- Windows Installation Instructions
- Linux Installation Instructions
- Appendix C. Answers to Quizzes and Exercises
- Hour 1, "Welcome to the World of SQL"
- Hour 2, "Defining Data Structures"
- Hour 3, "Managing Database Objects"
- Hour 4, "The Normalization Process"
- Hour 5, "Manipulating Data"
- Hour 6, "Managing Database Transactions"
- 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 Time"
- Hour 13, "Joining Tables in Queries"
- Hour 14, "Using Subqueries to Define Unknown Data"
- Hour 15, "Combining Multiple Queries into One"
- Hour 16, "Using Indexes to Improve Performance"
- Hour 17, "Improving Database Performance"
- Hour 18, "Managing Database Users"
- Hour 19, "Managing Database Security"
- Hour 20, "Creating and Using Views and Synonyms"
- Hour 21, "Working with the System Catalog"
- Hour 22, "Advanced SQL Topics"
- Hour 23, "Extending SQL to the Enterprise, the Internet, and the Intranet"
- Hour 24, "Extensions to Standard SQL"
- Appendix D. <tt>CREATE TABLE</tt> Statements for Book Examples
- <tt>EMPLOYEE_TBL</tt>
- <tt>EMPLOYEE_PAY_TBL</tt>
- <tt>CUSTOMER_TBL</tt>
- <tt>ORDERS_TBL</tt>
- <tt>PRODUCTS_TBL</tt>
- Appendix E. <tt>INSERT</tt> Statements for Data in Book Examples
- <tt>INSERT</tt> Statements
- Appendix F. Glossary
- Appendix G. Bonus Exercises
Recommended Book
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Join Considerations
Several things should be considered before using joins. Some considerations include what columns(s) to join on, whether there is no common column to join on, and performance issues. More joins in a query means the database server has to do more work, which means that more time is taken to retrieve data. Joins cannot be avoided when retrieving data from a normalized database, but it is imperative to ensure that joins are performed correctly from a logical standpoint. Incorrect joins can result in serious performance degradation and inaccurate query results. Performance issues are discussed in more detail in Hour 18, "Managing Database Users."
Using a BASE TABLE
What to join on? Should you have the need to retrieve data from two tables that do not have a common column to join, you must use another table that has a common column or columns to both tables to join on. That table becomes the BASE TABLE. A BASE TABLE is used to join one or more tables that have common columns, or to join tables that do not have common columns. Use the following three tables for an example of a base table:
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CUSTOMER_TBL |
|||
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CUST_ID |
VARCHAR(10) |
NOT NULL |
PRIMARY KEY |
|
CUST_NAME |
VARCHAR(30) |
NOT NULL |
|
|
CUST_ADDRESS |
VARCHAR(20) |
NOT NULL |
|
|
CUST_CITY |
VARCHAR(15) |
NOT NULL |
|
|
CUST_STATE |
VARCHAR(2) |
NOT NULL |
|
|
CUST_ZIP |
INTEGER(5) |
NOT NULL |
|
|
CUST_PHONE |
INYEGER(10) |
||
|
CUST_FAX |
INTEGER(10) |
|
ORDERS_TBL |
|||
|
ORD_NUM |
VARCHAR(10) |
NOT NULL |
PRIMARY KEY |
|
CUST_ID |
VARCHAR(10) |
NOT NULL |
|
|
PROD_ID |
VARCHAR(10) |
NOT NULL |
|
|
QTY |
INTEGER(6) |
NOT NULL |
|
|
ORD_DATE |
DATETIME |
|
PRODUCTS_TBL |
|||
|
PROD_ID |
VARCHAR(10) |
NOT NULL |
PRIMARY KEY |
|
PROD_DESC |
VARCHAR(40) |
NOT NULL |
|
|
COST |
DECIMAL(6,2) |
NOT NULL |
You have a need to use the CUSTOMERS_TBL and the PRODUCTS_TBL. There is no common column in which to join the tables. Now look at the ORDERS_TBL. The ORDERS_TBL has CUST_ID to join with CUSTOMERS_TBL, which also has CUST_ID. The PRODUCTS_TBL has PROD_ID, which is also in ORDERS_TBL. The JOIN conditions and results look like the following:
SELECT C.CUST_NAME, P.PROD_DESC FROM CUSTOMER_TBL C, PRODUCTS_TBL P, ORDERS_TBL O WHERE C.CUST_ID = O.CUST_ID AND P.PROD_ID = O.PROD_ID;
CUST_NAME PROD_DESC ------------------------------ ----------------------- LESLIE GLEASON WITCHES COSTUME SCHYLERS NOVELTIES PLASTIC PUMPKIN 18 INCH WENDY WOLF PLASTIC PUMPKIN 18 INCH GAVINS PLACE LIGHTED LANTERNS SCOTTYS MARKET FALSE PARAFFIN TEETH ANDYS CANDIES KEY CHAIN 6 rows selected.
The Cartesian Product
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The Cartesian product is a result of a CARTESIAN JOIN or "no join." If you select from two or more tables and do not JOIN the tables, your output is all possible rows from all the tables selected from. If your tables were large, the result could be hundreds of thousands, or even millions, of rows of data. A WHERE clause is highly recommended for SQL statements retrieving data from two or more tables. The Cartesian product is also known as a cross join. |
The syntax is
FROM TABLE1, TABLE2 [, TABLE3 ] WHERE TABLE1, TABLE2 [, TABLE3 ]
The following is an example of a cross join, or the dreaded Cartesian product:
Data is being selected from two separate tables, yet no JOIN operation is performed. Because you have not specified how to join rows in the first table with rows in the second table, the database server pairs every row in the first table with every row in the second table. Because each table has 6 rows of data each, the product of 36 rows selected is achieved from 6 rows multiplied by 6 rows.
To fully understand exactly how the Cartesian product is derived, study the following example.