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MySQL Cookbook
book

MySQL Cookbook

by Paul DuBois
October 2002
Intermediate to advanced content levelIntermediate to advanced
1024 pages
27h 26m
English
O'Reilly Media, Inc.
Content preview from MySQL Cookbook

Forcing MySQL to Treat Strings as Temporal Values

Problem

You want a string to be interpreted temporally.

Solution

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

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 ...
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Publisher Resources

ISBN: 0596001452Catalog PageErrata