MySQL in a Nutshell, 2nd Edition

Name

PERIOD_DIFF()

Synopsis

PERIOD_DIFF(yearmonth,yearmonth)

This function returns the number of months between the periods given. The periods given must be in string format and contain only the year and month, in either yyyymm or yymm format. Here is an example:

SELECT appointment AS 'Date of Appointment',
CURDATE( ) AS 'Current Date',
PERIOD_DIFF(
   EXTRACT(YEAR_MONTH FROM appointment),
   EXTRACT(YEAR_MONTH FROM CURDATE( ))
) AS 'Accounting Periods Apart';

+------------------+--------------+-----------------------------+
| Last Appointment | Current Date | Accounting Periods Ellapsed |
+------------------+--------------+-----------------------------+
| 2008-11-15       | 2009-01-15   |                          -2 | 
+------------------+--------------+-----------------------------+

This SQL statement determines that it has been two months since the client’s last appointment. If you want the results not to contain a negative, either switch the order of the periods or wrap the PERIOD_DIFF() within ABS(). The PERIOD_DIFF() function takes into account that the periods are in different years. But it doesn’t work on standard date columns, so you have to put them into the proper string format as shown here with a function such as EXTRACT().