Question

I know I’m going to go ‘duh’ when I see the answer to this, but nonetheless:

How do I group a date field by week, where the weeks start on Saturday?

Answer 1

You need to first use DATEFIRST to set Saturday as the first day of your week. Value 1 denotes Monday and value 6 denotes the day Saturday. Then you can use the DATEPART with week or wk or ww to group your date column values.

You can see in the below sample data that values for dates 2012-01-07 and 2012-01-08 are grouped under weekno 2 because 2012-01-07 is a Saturday. January 1 of any year is always first week of the year.

Click here to view the output in SQL Fiddle.

Script:

CREATE TABLE weekdays
(
    datevalue    datetime NOT NULL
  , numericvalue INT      NOT NULL
);

INSERT INTO weekdays (datevalue, numericvalue) VALUES
  ('2012-01-01', 4),
  ('2012-01-02', 1),
  ('2012-01-07', 2),
  ('2012-01-08', 8),
  ('2012-01-14', 3),
  ('2012-01-15', 2),
  ('2012-01-19', 6);

SET DATEFIRST 6;

SELECT      DATEPART(wk, datevalue)     weekno
        ,   SUM(numericvalue)           totalvalue
FROM        weekdays
GROUP BY    DATEPART(wk, datevalue)

Output:

weekno totalvalue
------ ----------
  1        5
  2       10
  3       11