5.26 Finding Dates for Days of the Current Week
5.26.1 Problem
You want to compute the date for
some other day of the current week.
5.26.2 Solution
Figure out the number of days between the starting day and the
desired day, and shift the date by that many days.
5.26.3 Discussion
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:
Shift the reference date back by its DAYOFWEEK( )
value, which always produces the date for the Saturday preceding the
week.
Add one day to the result to reach the Sunday date, two days to reach
the Monday date, and so forth.
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 |
+------------+----------+------------+------------+
|
