Question

The time is: (m/d/yyyy) => 2009/01/04

Using this command using datepart(wk,'20090104') I can get the week number (for any given date).

So :

SELECT datepart(wk,'20090101') //1
SELECT datepart(wk,'20090102') //1
SELECT datepart(wk,'20090103') //1
SELECT datepart(wk,'20090104') //2

So far so good.

The problem :

Those 3 first dates are not part of a full week, so I can’t put them in a fixed 52-week chart.

Our company needs to see information about each whole week in the 52 weeks of a year. (Each year has 52 whole weeks).

So 20090101 doesn’t belong to the first week of 2009 !

It belongs to the previous year (which is irrelevant to my question)

So I need a UDF (I’ve been searching a lot, and ISOWEEK is not answering my needs) which by a given datetime, will give me the Week Number (week = whole week, so partial weeks aren’t considered).

Example :

calcweekNum ('20090101') //52 ...from the last year
calcweekNum ('20090102') //52 ...from the last year
calcweekNum ('20090103') //52 ...from the last year
calcweekNum ('20090104') //1
..
..
calcweekNum ('20090110') //1
calcweekNum ('20090111') //2
calcweekNum ('20090112') //2
...

Answer 1

Here’s a different approach. All you need to supply is the year:

DECLARE @year INT = 2009;


DECLARE @start SMALLDATETIME;
SET @start = DATEADD(YEAR, @year-1900, 0);

;WITH n AS
(
  SELECT TOP (366) -- in case of leap year
      d = DATEADD(DAY, ROW_NUMBER() OVER (ORDER BY name)-1, @start)
    FROM sys.all_objects
),
x AS 
(
  SELECT md = MIN(d) FROM n 
    WHERE DATEPART(WEEKDAY, d) = 1 -- assuming DATEFIRST is Sunday
),
y(d,wk) AS
(
  SELECT n.d, ((DATEPART(DAYOFYEAR, n.d) - DATEDIFF(DAY, @start, x.md)-1)/7) + 1
  FROM n CROSS JOIN x
  WHERE n.d >= x.md
  AND n.d < DATEADD(YEAR, 1, @start)
)
SELECT [date] = d, [week] = wk
FROM y WHERE wk < 53
ORDER BY [date];

Results:

date        week
----------  ----
2009-01-04  1
2009-01-05  1
2009-01-06  1
2009-01-07  1
2009-01-08  1
2009-01-09  1
2009-01-10  1
2009-01-11  2
2009-01-12  2
...
2009-12-25  51
2009-12-26  51
2009-12-27  52
2009-12-28  52
2009-12-29  52
2009-12-30  52
2009-12-31  52

Note that week 52 won’t necessarily be a full week, and that in some cases (e.g. 2012), the last day or two of the year might fall in week 53, so they’re excluded.

An alternative approach is to repeat the MIN expression twice:

DECLARE @year INT = 2009;


DECLARE @start SMALLDATETIME;
SET @start = DATEADD(YEAR, @year-1900, 0);

;WITH n AS
(
  SELECT TOP (366) -- in case of leap year
      d = DATEADD(DAY, ROW_NUMBER() OVER (ORDER BY name)-1, @start)
    FROM sys.all_objects
),
y(d,wk) AS
(
  SELECT n.d, ((DATEPART(DAYOFYEAR, n.d) - DATEDIFF(DAY, @start, (SELECT MIN(d) 
    FROM n WHERE DATEPART(WEEKDAY, d) = 1))-1)/7) + 1
  FROM n
  WHERE n.d >= (SELECT md = MIN(d) FROM n WHERE DATEPART(WEEKDAY, d) = 1)
  AND n.d < DATEADD(YEAR, 1, @start)
)
SELECT [date] = d, [week] = wk
FROM y WHERE wk < 53
ORDER BY d;