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
    • 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
  • 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

Date Functions

Date functions are available in SQL depending on the options with each specific implementation. Date functions, similar to character string functions, are used to manipulate the representation of date and time data. Available date functions are often used to format the output of dates and time in an appealing format, compare date values with one another, compute intervals between dates, and so on.

The Current Date

You may have already raised the question: How do I get the current date from the database? The need to retrieve the current date from the database may originate from several situations, but the current date is normally returned either to compare to a stored date or to return the value of the current date as some sort of timestamp.

The current date is ultimately stored on the host computer for the database, and is called the system date. The database, which interfaces with the appropriate operating system, has the capability to retrieve the system date for its own purpose or to resolve database requests, such as queries.

Take a look at a couple of methods of attaining the system date based on commands from two different implementations.

Sybase uses a function called GETDATE() to return the system date. This function is used in a query as follows. The output is what would return if today's current date were New Year's Eve for 1999.

   
   
   

   SELECT GETDATE()

   
Dec 31, 1999

Oracle uses what is calls a pseudocolumn, SYSDATE, to retrieve the current date. SYSDATE acts as any other column in a table and can be selected from any table in the database, although it is not actually part of the table's definition.

To return the system date in Oracle, the following statement returns the output if today were New Year's Eve before 2002:

   
   

   SELECT SYSDATE FROM TABLE_NAME

   
31-DEC-01

Time Zones

The use of time zones may be a factor when dealing with date and time information. For instance, a time of 6:00 p.m. in central United States does not equate to the same time in Australia, although the actual point in time is the same. Some of us who live within the daylight saving time zone are used to adjusting our clocks twice a year. If time zones are considerations when maintaining data in your case, you may find it necessary to consider time zones and perform time conversions, if available with your SQL implementation.

The following are some common time zones and their abbreviations:

The following table shows examples of time zone differences based on a given time:

Adding Time to Dates

Days, months, and other parts of time can be added to dates for the purpose of comparing dates to one another, or to provide more specific conditions in the WHERE clause of a query.

Intervals can be used to add periods of time to a DATETIME value. As defined by the standard, intervals are used to manipulate the value of a DATETIME value, as in the following examples:

   
   

   DATE '1999-12-31' + INTERVAL '1' DAY

   
'2000-01-01'


   DATE '1999-12-31' + INTERVAL '1' MONTH

   
'2000-01-31'

The following is an example using the SQL Server function DATEADD:

   

   SELECT DATEADD(MONTH, 1, DATE_HIRE)

   FROM EMPLOYEE_PAY_TBL;

   
DATE_HIRE ADD_MONTH
--------- ---------
23-MAY-89 23-JUN-89
17-JUN-90 17-JUL-90
14-AUG-94 14-SEP-94
28-JUN-97 28-JUL-97
22-JUL-96 22-AUG-96
14-JAN-91 14-FEB-91

6 rows affected.

The following example uses the Oracle function ADD_MONTHS:

   
   

   SELECT DATE_HIRE, ADD_MONTHS(DATE_HIRE,1)

   FROM EMPLOYEE_PAY_TBL;

   
DATE_HIRE ADD_MONTH
--------- ---------
23-MAY-89 23-JUN-89
17-JUN-90 17-JUL-90
14-AUG-94 14-SEP-94
28-JUN-97 28-JUL-97
22-JUL-96 22-AUG-96
14-JAN-91 14-FEB-91

6 rows selected.

To add one day to a date in Oracle, use the following:

   

   SELECT DATE_HIRE, DATE_HIRE + 1

   FROM EMPLOYEE_PAY_TBL

   WHERE EMP_ID = '311549902';

   
DATE_HIRE DATE_HIRE
--------- ---------
23-MAY-89 24-MAY-89

1 row selected.

Notice that these examples in SQL Server and Oracle, although they differ syntactically from the ANSI examples, derive their results based on the same concept as described by the SQL standard.

Comparing Dates and Time Periods

OVERLAPS is a powerful standard SQL conditional operator for DATETIME values. The OVERLAPS operator is used to compare two timeframes and return the Boolean value TRUE or FALSE, depending on whether the two timeframes overlap. The following comparison returns the value TRUE:

(TIME '01:00:00' , TIME '05:59:00') 
OVERLAPS
(TIME '05:00:00' , TIME '07:00:00')

The following comparison returns the value FALSE:

(TIME '01:00:00' , TIME '05:59:00') 
OVERLAPS
(TIME '06:00:00 , TIME '07:00:00')

Miscellaneous Date Functions

The following list shows some powerful date functions that exist in the implementations for SQL Server, Oracle, and MySQL.

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