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 |
+------------+------------+------------+------------+------------+

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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