Recipe 6.14. Calculating Dates by Substring Replacement
ProblemGiven a date, you want to produce another date from it when you know that the two dates share some components in common. SolutionTreat a date or time value as a string, and perform direct replacement on parts of the string. DiscussionIn 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 statement shows two ways to determine the date for Christmas two years hence. The first method finds Christmas for this year, and then shifts it two years forward. The second shifts the current date forward two years, and 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 | +------------+------------+------------+ | 2006-05-22 | 2008-12-25 | 2008-12-25 | +------------+------------+------------+ |
