Forcing MySQL to Treat Strings as Temporal Values

5.30.1 Problem

You want a string to be interpreted temporally.

5.30.2 Solution

Use the string in a temporal context to give MySQL a hint about how to treat it.

5.30.3 Discussion

If you need to make MySQL treat a string as a date or time, use it in an expression that provides a temporal context without changing the value. For example, you can't add zero to a literal TIME string to cause a time-to-number conversion, but if you use TIME_TO_SEC( ) and SEC_TO_TIME( ), you can:

mysql> SELECT SEC_TO_TIME(TIME_TO_SEC('12:30:45'))+0;
+----------------------------------------+
| SEC_TO_TIME(TIME_TO_SEC('12:30:45'))+0 |
+----------------------------------------+
| 123045 |
+----------------------------------------+

The conversion to and from seconds leaves the value unchanged but results in a context where MySQL treats the result as a TIME value. For date values, the procedure is similar, but uses TO_DAYS( ) and FROM_DAYS( ):

mysql> SELECT '1999-01-01'+0, FROM_DAYS(TO_DAYS('1999-01-01'))+0;
+----------------+------------------------------------+
| '1999-01-01'+0 | FROM_DAYS(TO_DAYS('1999-01-01'))+0 |
+----------------+------------------------------------+
| 1999 | 19990101 |
+----------------+------------------------------------+

For DATETIME- or TIMESTAMP-formatted strings, you can use DATE_ADD( ) to introduce a temporal context:

mysql> SELECT
 -> DATE_ADD('1999-01-01 12:30:45',INTERVAL 0 DAY)+0 AS 'numeric datetime',
 -> DATE_ADD('19990101123045',INTERVAL 0 DAY)+0 AS 'numeric timestamp';
+------------------+-------------------+
| numeric datetime | numeric timestamp |
+------------------+-------------------+
| 19990101123045 | 19990101123045 |
+------------------+-------------------+

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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