Calculating One Date from Another by Substring Replacement

5.24.1 Problem

Given a date, you want to produce another date from it, and you know the two dates share some components in common.

5.24.2 Solution

Treat a date or time value as a string and perform direct replacement on parts of the string.

5.24.3 Discussion

In some cases, you can use substring replacement to calculate dates without performing any date arithmetic. For example, you can use string operations to produce the first-of-month value for a given date by replacing the day component with 01. You can do this either with DATE_FORMAT( ) or with CONCAT( ):

mysql> SELECT d,
 -> DATE_FORMAT(d,'%Y-%m-01') AS method1,
 -> CONCAT(YEAR(d),'-',LPAD(MONTH(d),2,'0'),'-01') AS method2
 -> FROM date_val;
+------------+------------+------------+
| d | method1 | method2 |
+------------+------------+------------+
| 1864-02-28 | 1864-02-01 | 1864-02-01 |
| 1900-01-15 | 1900-01-01 | 1900-01-01 |
| 1987-03-05 | 1987-03-01 | 1987-03-01 |
| 1999-12-31 | 1999-12-01 | 1999-12-01 |
| 2000-06-04 | 2000-06-01 | 2000-06-01 |
+------------+------------+------------+

The string replacement technique can also be used to produce dates with a specific position within the calendar year. For New Year's Day (January 1), replace the month and day with 01:

mysql> SELECT d,
 -> DATE_FORMAT(d,'%Y-01-01') AS method1,
 -> CONCAT(YEAR(d),'-01-01') AS method2
 -> FROM date_val;
+------------+------------+------------+
| d | method1 | method2 |
+------------+------------+------------+
| 1864-02-28 | 1864-01-01 | 1864-01-01 |
| 1900-01-15 | 1900-01-01 | 1900-01-01 |
| 1987-03-05 | 1987-01-01 | 1987-01-01 |
| 1999-12-31 | 1999-01-01 | 1999-01-01 |
| 2000-06-04 | 2000-01-01 | 2000-01-01 |
+------------+------------+------------+

For Christmas, replace the month and day with 12 and 25:

mysql> SELECT d,
 -> DATE_FORMAT(d,'%Y-12-25') AS method1,
 -> CONCAT(YEAR(d),'-12-25') AS method2
 -> FROM date_val;
+------------+------------+------------+
| d | method1 | method2 |
+------------+------------+------------+
| 1864-02-28 | 1864-12-25 | 1864-12-25 |
| 1900-01-15 | 1900-12-25 | 1900-12-25 |
| 1987-03-05 | 1987-12-25 | 1987-12-25 |
| 1999-12-31 | 1999-12-25 | 1999-12-25 |
| 2000-06-04 | 2000-12-25 | 2000-12-25 |
+------------+------------+------------+

To perform the same operation for Christmas in other years, combine string replacement with date shifting. The following query shows two ways to determine the date for Christmas two years hence. The first method finds Christmas for this year, then shifts it two years forward. The second shifts the current date forward two years, then finds Christmas in the resulting year:

mysql> SELECT CURDATE( ),
 -> DATE_ADD(DATE_FORMAT(CURDATE( ),'%Y-12-25'),INTERVAL 2 YEAR)
 -> AS method1,
 -> DATE_FORMAT(DATE_ADD(CURDATE( ),INTERVAL 2 YEAR),'%Y-12-25')
 -> AS method2;
+------------+------------+------------+
| CURDATE( ) | method1 | method2 |
+------------+------------+------------+
| 2002-07-15 | 2004-12-25 | 2004-12-25 |
+------------+------------+------------+

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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