Sams Teach Yourself SQL in 24 Hours
- 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
- 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
Comparison Operators
Comparison operators are used to test single values in a SQL statement. The comparison operators discussed consist of =, <>, <, and >.
These operators are used to test
- Equality
- Non-equality
- Less-than values
- Greater-than values
Examples and the meanings of comparison operators are covered in the following sections.
Equality
The equal operator compares single values to one another in a SQL statement. The equal sign (=) symbolizes equality. When testing for equality, the compared values must match exactly or no data is returned. If two values are equal during a comparison for equality, the returned value for the comparison is TRUE; the returned value is FALSE if equality is not found. This Boolean value (TRUE/FALSE) is used to determine whether data is returned according to the condition.
The = operator can be used by itself or combined with other operators. An example and the meaning of the equality operator follows:
|
Example |
Meaning |
|
WHERE SALARY = '20000' |
Salary equals 20000 |
The following query returns all rows of data where the PROD_ID is equal to 2345:
SELECT * FROM PRODUCTS_TBL WHERE PROD_ID = '2345';
PROD_ID PROD_DESC COST ---------- ------------------------------ ------ 2345 OAK BOOKSHELF 59.99 1 row selected.
Non-Equality
For every equality, there are multiple non-equalities. In SQL, the operator used to measure non-equality is <> (the less-than sign combined with the greater-than sign). The condition returns TRUE if the condition finds non-equality; FALSE is returned if equality is found.
|
Example |
Meaning |
|
WHERE SALARY <> '20000' |
Salary does not equal 20000 |
Less-Than, Greater-Than
The symbols < (less-than) and > (greater-than) can be used by themselves, or in combination with each other or other operators.
|
Example |
Meaning |
|
WHERE SALARY < '20000' |
Salary is less than 20000 |
|
WHERE SALARY > '20000' |
Salary is greater than 20000 |
In the first example, anything less-than and not equal to 20000 returns TRUE. Any value of 20000 or more returns FALSE. Greater-than works the opposite of less-than.
In the next example, notice that the value 24.99 was not included in the query's result set. The less-than operator is not inclusive.
Combination Examples of Comparison Operators
The equal operator can be combined with the less-than and greater-than operators, as in the following examples:
|
Example |
Meaning |
|
WHERE SALARY <= '20000' |
Salary less-than or equal-to |
|
WHERE SALARY >= '20000' |
Salary greater-than or equal-to |
Less-than or equal-to 20000 includes 20000 and all values less than 20000. Any value in that range returns TRUE; any value greater than 20000 returns FALSE. Greater-than or equal-to also includes the value 20000 in this case and works the same as the less-than or equal-to.
Logical Operators | Next Section
Account Sign In
View your cart