About the Author
Russell Dyer has worked full-time for several years as a free-lance writer of computer articles, primarily on MySQL. He has been working with MySQL AB as the editor of their new Knowledge Base since December 2004. He is the author of "MySQL in a Nutshell" (O'Reilly), and has published over ninety articles for several magazines: Dev Zone (a MySQL publication), Linux Journal, ONlamp.com, The Perl Journal, Red Hat Magazine, SysAdmin Magazine, Tech Republic, Unix Review, and XML.com. A list of his published articles with links to them can be found on his web site at http://russell.dyerhouse.com.
Excerpt. © Reprinted by permission. All rights reserved.
The ability to record dates and times in a MySQL database is a very common requirement. This chapter presents the date and time functions for MySQL.
Date and time data comprises only numeric strings, so it can be stored in a regular character column. However, by using temporal datatype columns, you can use several built-in functions offered by MySQL. Currently, five temporal datatypes are available: date, time, datetime, timestamp, and year. The date column type is only for recording the date and uses the format yyyy-mm-dd. The time column type is for recording time in the format hhh:mm:ss. To record a combination of date and time, you can use the datetime column type: yyyy-mm-dd hh:mm:ss. The timestamp column is similar to datetime, but is a little limited in its range of allowable time: it starts at the Unix epoch time (i.e., 1970-01-01) and ends at the end of 2037. Finally, the year datatype is used only for recording the year in a column.
Incidentally, any function that calls for a date or a time datatype will also accept a combined datetime datatype. For more information on date and time datatypes, see Appendix A.
Validation of date strings is limited: MySQL makes sure that months range only from 0 to 12, and days range from 0 to 31. Therefore, a date such as February 30would be accepted. Version 5.0.2 of MySQL will offer more refined validation that would reject such a date.
At the end of this introduction is a listing of date and time functions, grouped by type of function. The bulk of this chapter consists of an alphabetical listing of date and time functions, with explanations of each. Many functions come with examples, along with a resulting display. For help in locating functions, see the index at the back of this book.
For the examples in this chapter, I used the scenario of a professional services firm (e.g., a law firm or an investment advisory firm) that tracks appointments and seminars in MySQL.
Date and Time Functions Grouped by Type
This section lists the functions according to their purpose: to retrieve a time, extract an element of one, or perform calculations on it.
Determining the Date and Time
CURDATE( ), CURRENT_DATE, CURTIME( ), CURRENT_TIME, CURRENT_TIMESTAMP, NOW( ),LOCALTIME( ), LOCALTIMESTAMP( ), SYSDATE( ), UNIX_TIMESTAMP( ), UTC_DATE( ), UTC_TIME( ), UTC_TIMESTAMP( )
Extracting and Formatting the Date and Time
DATE( ), DATE_FORMAT( ), DAY( ), DAYNAME( ), DAYOFMONTH( ), DAYOFWEEK( ), DAYOFYEAR( ), EXTRACT( ), GET_FORMAT( ), HOUR( ), LAST_DAY( ), MAKEDATE( ), MAKETIME( ), MINUTE( ), MONTH( ), MONTHNAME( ), QUARTER( ), SECOND( ), STR_TO_DATE( ), TIME_FORMAT( ), TIMESTAMP( ), WEEK( ), WEEKDAY( ), WEEKOFYEAR( ), YEAR( ), YEARWEEK( )
Calculating and Modifying the Date and Time
ADDDATE( ), ADDTIME( ), CONVERT_TZ( ), DATE_ADD( ), DATE_SUB( ), DATEDIFF( ), FROM_DAYS( ), FROM_UNIXTIME( ), PERIOD_ADD( ), PERIOD_DIFF( ), SEC_TO_TIME( ), SUBDATE( ), SUBTIME( ), TIME_TO_SEC( ), TO_DAYS( ), TIMEDIFF( ), TIMESTAMPADD( ), TIMESTAMPDIFF( )
Date and Time Functions in Alphabetical Order
The rest of the chapter lists each function in alphabetical order.
ADDDATE(date,INTERVAL value type)
This function adds the given interval of time to the date or time provided. This is an alias for DATE_ADD( ); see its definition for details and interval types.
SET seminar_date = ADDDATE(seminar_date, INTERVAL 7 DAY)
WHERE seminar_date = '2004-12-15'';
This example postpones the seminar that was scheduled for December 15, 2004 to December 22seven days later. As of Version 4.1 of MySQL, for adding days the second argument of the function may simply be the number of days (i.e., just 7 instead of INTERVAL 7 DAY).