How do I get the first day of the week of a date in mysql?

Question

Suppose I have 2011-01-03 and I want to get the first of the week, which is sunday, which is 2011-01-02, how do I go about doing that?

The reason is I have this quer

Answer 1

If you need to handle weeks which start on Mondays, you could also do it that way. First define a custom FIRST_DAY_OF_WEEK function:

DELIMITER ;;
CREATE FUNCTION FIRST_DAY_OF_WEEK(day DATE)
RETURNS DATE DETERMINISTIC
BEGIN
  RETURN SUBDATE(day, WEEKDAY(day));
END;;
DELIMITER ;

And then you could do:

SELECT FIRST_DAY_OF_WEEK('2011-01-03');

For your information, MySQL provides two different functions to retrieve the first day of a week. There is DAYOFWEEK:

Returns the weekday index for date (1 = Sunday, 2 = Monday, …, 7 = Saturday). These index values correspond to the ODBC standard.

And WEEKDAY:

Returns the weekday index for date (0 = Monday, 1 = Tuesday, … 6 = Sunday).

Answer 2

If the week starts on Monday do this:

DATE_SUB(mydate, INTERVAL WEEKDAY(mydate) DAY)