Artificial Intelligence - MySQL

When dealing with date and time values, one of the more common (and complex) tasks involves performing addition and subtraction operations on these values. However, with MySQL's powerful date and time API taking care of all the minor adjustments for you, manipulating date and time data is no longer the tedious and time-consuming process it used to be. Find out why, inside.

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By: icarus, (c) Melonfire

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July 03, 2003

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It should be noted that MySQL includes intelligence to automatically format the result of the calculation as either a date-only or date-and-time value. For example, if your arguments to DATE_ADD() or DATE_SUB() contains only year, month and day components, MySQL will output a date-only value as result.



mysql> SELECT DATE_SUB(20041130, INTERVAL 3 MONTH);

+--------------------------------------+

| DATE_SUB(20041130, INTERVAL 3 MONTH) |

+--------------------------------------+

| 2004-08-30 |

+--------------------------------------+

1 row in set (0.00 sec)

However, if your arguments include a time component - hours, minutes or seconds - then MySQL produces a result containing both date and time components.



mysql> SELECT DATE_ADD('2010-02-14', INTERVAL "1 1" DAY_HOUR);

+-------------------------------------------------+

| DATE_ADD('2010-02-14', INTERVAL "1 1" DAY_HOUR) |

+-------------------------------------------------+

| 2010-02-15 01:00:00 |

+-------------------------------------------------+

1 row in set (0.01 sec)

If you use illegal date or time values, MySQL will still attempt to return a valid result by performing adjustments on the various values.



mysql> SELECT DATE_ADD('2010-02-30', INTERVAL 1 DAY);

+----------------------------------------+

| DATE_ADD('2010-02-30', INTERVAL 1 DAY) |

+----------------------------------------+

| 2010-03-03 |

+----------------------------------------+

1 row in set (0.00 sec)

MySQL also provides an alternative syntax to DATE_ADD() and DATE_SUB(), which is sometimes more readable - this involves using + and - signs to indicate the type of calculation to be performed. Consider the
following examples, which illustrate:



mysql> SELECT 20000615 + INTERVAL 5 DAY;

+---------------------------+

| 20000615 + INTERVAL 5 DAY |

+---------------------------+

| 2000-06-20 |

+---------------------------+

1 row in set (0.00 sec)
mysql> SELECT 20000615000000 - INTERVAL 10 SECOND ;

+-------------------------------------+

| 20000615000000 - INTERVAL 10 SECOND |

+-------------------------------------+

| 2000-06-14 23:59:50 |

+-------------------------------------+

1 row in set (0.00 sec)
mysql> SELECT '2000-06-15 13:00' - INTERVAL "2-1" YEAR_MONTH;

+------------------------------------------------+

| '2000-06-15 13:00' - INTERVAL "2-1" YEAR_MONTH |

+------------------------------------------------+

| 1998-05-15 13:00:00 |

+------------------------------------------------+

1 row in set (0.00 sec)