You want to compute the date for some other day of the current week.
Figure out the number of days between the starting day and the desired day, and shift the date by that many days.
This section and the next describe how to convert one date to another when the target date is specified in terms of days of the week. To solve such problems, you need to know day-of-week values. For example, if you want to know what date it is on Tuesday of this week, the calculation depends on what day of the week it is today. If today is Monday, you add a day to CURDATE( ), but if today is Wednesday, you subtract a day.
MySQL provides two functions that are useful here. DAYOFWEEK( ) treats Sunday as the first day of the week and returns 1 through 7 for Sunday through Saturday. WEEKDAY( ) treats Monday as the first day of the week and returns 0 through 6 for Monday through Sunday. (The examples shown here use DAYOFWEEK( ).) Another kind of day-of-week operation involves determining the name of the day. DAYNAME( ) can be used for that.
Calculations that determine one day of the week from another depend on the day you start from as well as the day you want to reach. I find it easiest to shift the reference date first to a known point relative to the beginning of the week, then shift forward:
In SQL, these operations can be expressed as follows for a date d, where n is 1 through 7 to produce the dates for Sunday through Saturday:
DATE_ADD(DATE_SUB(d,INTERVAL DAYOFWEEK(d) DAY),INTERVAL n DAY)
That expression splits the "shift back to Saturday" and "shift forward" phases into separate operations, but because the intervals for both DATE_SUB( ) and DATE_ADD( ) are both in days, the expression can be simplified into a single DATE_ADD( ) call:
DATE_ADD(d,INTERVAL n-DAYOFWEEK(d) DAY)
If we apply this formula to our date_val table, using an n of 1 for Sunday and 7 for Saturday to find the first and last days of the week, we get this result:
mysql> SELECT d, DAYNAME(d) AS day, -> DATE_ADD(d,INTERVAL 1-DAYOFWEEK(d) DAY) AS Sunday, -> DATE_ADD(d,INTERVAL 7-DAYOFWEEK(d) DAY) AS Saturday -> FROM date_val; +------------+----------+------------+------------+ | d | day | Sunday | Saturday | +------------+----------+------------+------------+ | 1864-02-28 | Sunday | 1864-02-28 | 1864-03-05 | | 1900-01-15 | Monday | 1900-01-14 | 1900-01-20 | | 1987-03-05 | Thursday | 1987-03-01 | 1987-03-07 | | 1999-12-31 | Friday | 1999-12-26 | 2000-01-01 | | 2000-06-04 | Sunday | 2000-06-04 | 2000-06-10 | +------------+----------+------------+------------+
Using the mysql Client Program
Writing MySQL-Based Programs
Record Selection Techniques
Working with Strings
Working with Dates and Times
Sorting Query Results
Modifying Tables with ALTER TABLE
Obtaining and Using Metadata
Importing and Exporting Data
Generating and Using Sequences
Using Multiple Tables
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