MySQL Cookbook, 2nd Edition

Calculating Dates by Substring Replacement

Problem

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

Solution

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

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 ...