5.24 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 |
+------------+------------+------------+
| 
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