5.19 Canonizing Not-Quite-ISO Date Strings
5.19.1 Problem
A date is in a format
that's close to but not exactly ISO format.
5.19.2 Solution
Canonize the date by passing it to a function that always returns an
ISO-format date result.
5.19.3 Discussion
Earlier in the chapter (Recipe 5.9), we ran into the problem that synthesizing
dates with CONCAT( ) may produce values that are
not quite in ISO format. For example, the following query produces
first-of-month values in which the month part may have only a single
digit:
mysql> SELECT d, CONCAT(YEAR(d),'-',MONTH(d),'-01') FROM date_val;
+------------+------------------------------------+
| d | CONCAT(YEAR(d),'-',MONTH(d),'-01') |
+------------+------------------------------------+
| 1864-02-28 | 1864-2-01 |
| 1900-01-15 | 1900-1-01 |
| 1987-03-05 | 1987-3-01 |
| 1999-12-31 | 1999-12-01 |
| 2000-06-04 | 2000-6-01 |
+------------+------------------------------------+
In that section, a technique using LPAD(
) was shown for making
sure the month values have two digits.
mysql> SELECT d, CONCAT(YEAR(d),'-',LPAD(MONTH(d),2,'0'),'-01') FROM date_val;
+------------+------------------------------------------------+
| d | CONCAT(YEAR(d),'-',LPAD(MONTH(d),2,'0'),'-01') |
+------------+------------------------------------------------+
| 1864-02-28 | 1864-02-01 |
| 1900-01-15 | 1900-01-01 |
| 1987-03-05 | 1987-03-01 |
| 1999-12-31 | 1999-12-01 |
| 2000-06-04 | 2000-06-01 |
+------------+------------------------------------------------+
Another way to standardize a close-to-ISO date is to use it in an
expression that produces an ISO date result. For a date
d, any of the following expressions will do:
DATE_ADD(d,INTERVAL 0 DAY)
d + INTERVAL 0 DAY
FROM_DAYS(TO_DAYS(d))
For example, the non-ISO results from the CONCAT(
) operation can be converted into ISO format three
different ways as follows:
mysql> SELECT d,
-> CONCAT(YEAR(d),'-',MONTH(d),'-01') AS 'non-ISO',
-> DATE_ADD(CONCAT(YEAR(d),'-',MONTH(d),'-01'),INTERVAL 0 DAY) AS method1,
-> CONCAT(YEAR(d),'-',MONTH(d),'-01') + INTERVAL 0 DAY AS method2,
-> FROM_DAYS(TO_DAYS(CONCAT(YEAR(d),'-',MONTH(d),'-01'))) AS method3
-> FROM date_val;
+------------+------------+------------+------------+------------+
| d | non-ISO | method1 | method2 | method3 |
+------------+------------+------------+------------+------------+
| 1864-02-28 | 1864-2-01 | 1864-02-01 | 1864-02-01 | 1864-02-01 |
| 1900-01-15 | 1900-1-01 | 1900-01-01 | 1900-01-01 | 1900-01-01 |
| 1987-03-05 | 1987-3-01 | 1987-03-01 | 1987-03-01 | 1987-03-01 |
| 1999-12-31 | 1999-12-01 | 1999-12-01 | 1999-12-01 | 1999-12-01 |
| 2000-06-04 | 2000-6-01 | 2000-06-01 | 2000-06-01 | 2000-06-01 |
+------------+------------+------------+------------+------------+
|
