Adding a Temporal Interval to a Time

5.14.1 Problem

You want to add a given number of seconds to a time, or to add two time values.

5.14.2 Solution

Use TIME_TO_SEC( ) as necessary to make sure all values are represented in seconds, then add them. The result will be in seconds; use SEC_TO_TIME( ) if you want to convert back to a time value.

5.14.3 Discussion

The primary tools for performing time arithmetic are TIME_TO_SEC( ) and SEC_TO_TIME( ), which convert between TIME values and seconds. To add an interval value in seconds to a TIME value, convert the TIME to seconds so that both values are represented in the same units, add the values together, and convert the result back to a TIME. For example, two hours is 7200 seconds (2*60*60), so the following query adds two hours to each t1 value in the time_val table:

mysql> SELECT t1,
 -> SEC_TO_TIME(TIME_TO_SEC(t1) + 7200) AS 't1 plus 2 hours'
 -> FROM time_val;
+----------+-----------------+
| t1 | t1 plus 2 hours |
+----------+-----------------+
| 15:00:00 | 17:00:00 |
| 05:01:30 | 07:01:30 |
| 12:30:20 | 14:30:20 |
+----------+-----------------+

If the interval itself is expressed as a TIME, it too should be converted to seconds before adding the values together. The following example calculates the sum of the two TIME values in the time_val table:

mysql> SELECT t1, t2,
 -> SEC_TO_TIME(TIME_TO_SEC(t1) + TIME_TO_SEC(t2)) AS 't1 + t2'
 -> FROM time_val;
+----------+----------+----------+
| t1 | t2 | t1 + t2 |
+----------+----------+----------+
| 15:00:00 | 15:00:00 | 30:00:00 |
| 05:01:30 | 02:30:20 | 07:31:50 |
| 12:30:20 | 17:30:45 | 30:01:05 |
+----------+----------+----------+

It's important to recognize that MySQL TIME values really represent elapsed time, not time of day, so they don't reset to 0 after reaching 24 hours. You can see this in the first and third output rows from the previous query. To produce time-of-day values, enforce a 24-hour wraparound using a modulo operation before converting the seconds value back to a TIME value. The number of seconds in a day is 24*60*60, or 86400, so to convert any seconds value s to lie within a 24-hour range, use the MOD( ) function or the % modulo operator like this:

MOD(s,86400)
s % 86400

The two expressions are equivalent. Applying the first of them to the time calculations from the preceding example produces the following result:

mysql> SELECT t1, t2,
 -> SEC_TO_TIME(MOD(TIME_TO_SEC(t1) + TIME_TO_SEC(t2), 86400)) AS 't1 + t2'
 -> FROM time_val;
+----------+----------+----------+
| t1 | t2 | t1 + t2 |
+----------+----------+----------+
| 15:00:00 | 15:00:00 | 06:00:00 |
| 05:01:30 | 02:30:20 | 07:31:50 |
| 12:30:20 | 17:30:45 | 06:01:05 |
+----------+----------+----------+

The allowable range of TIME values is -838:59:59 to 838:59:59 (that is -3020399 to 3020399 seconds). When you add times together, you can easily produce a result that lies outside this range. If you try to store such a value into a TIME column, MySQL clips it to the nearest endpoint of the range.

Using the mysql Client Program

Writing MySQL-Based Programs

Record Selection Techniques

Working with Strings

Working with Dates and Times

Sorting Query Results

Generating Summaries

Modifying Tables with ALTER TABLE

Obtaining and Using Metadata

Importing and Exporting Data

Generating and Using Sequences

Using Multiple Tables

Statistical Techniques

Handling Duplicates

Performing Transactions

Introduction to MySQL on the Web

Incorporating Query Resultsinto Web Pages

Processing Web Input with MySQL

Using MySQL-Based Web Session Management

Appendix A. Obtaining MySQL Software

Appendix B. JSP and Tomcat Primer

Appendix C. References



MySQL Cookbook
MySQL Cookbook
ISBN: 059652708X
EAN: 2147483647
Year: 2005
Pages: 412
Authors: Paul DuBois

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