Find the next occurance of a day of the week in SQL - sql-server-2005

Question

I'm trying to update a SQL report sproc's WHERE clause to check whether a given date falls on or before the next occurrence of a class. Classes have a StartDate and occur once per week on the same day each week. Given the StartDate, how can I find the next occurrence of that day of the week?
E.G. If the StartDate is 1/18/2012, a Wednesday, and I run the report as of today, 1/26/2012, I need to find 2/1/2012 which is the next Wednesday after 1/26. If the StartDate is 1/19, a Thurs, and I run the report today, the formula should give me Thurs 1/26 which is today.
Here's sort of the idea in SQL:
SELECT *
FROM tbl_Class cs
INNER JOIN tbl_Enrollment sce ON cs.pk_ClassID = sce.fk_ClassID
WHERE ...
AND sce.StartDate < [Find date of next class after #AsOfDate using cs.StartDate]

Answer 1

Here's some example SQL that I came up with. 3 iterations so you can follow how I got to the end. The 3rd iteration should be something you can incorporate into a WHERE clause by substituting your column names for the variables.
Setup:
DECLARE #Startdate DATETIME,#currentdate datetime
SET #Startdate = '1-26-2012'
SET #Currentdate = '1-23-2012'
--This section just normalizes it so you can use 7 as the interval
--The offset depends on your current setting for DATEFIRST, U.S. English default is 7, Sunday.
-- see http://msdn.microsoft.com/en-us/library/ms187766.aspx
DECLARE #StartDateWorkingDayOfWeek int,#CurrentDateWorkingDayOfWeek int
SELECT #StartDateWorkingDayOfWeek =(DATEPART(weekday,#Startdate)-2)
SELECT #CurrentDateWorkingDayOfWeek=(DATEPART(weekday,#Currentdate)-2)
Iteration #1
--Iteration 1
IF #StartDateWorkingDayOfWeek < #CurrentDateWorkingDayOfWeek
SELECT DATEADD(DAY,DATEDIFF(DAY,0,#Currentdate)/7*7 + 7,#StartDateWorkingDayOfWeek)
else
SELECT DATEADD(DAY,DATEDIFF(DAY,0,#Currentdate)/7*7 + 0,#StartDateWorkingDayOfWeek)
Iteration #2
--Iteration 2
SELECT DATEADD(DAY,DATEDIFF(DAY,0,#Currentdate)/7*7 +
CASE WHEN #StartDateWorkingDayOfWeek < #CurrentDateWorkingDayOfWeek
then 7
ELSE 0
end
,#StartDateWorkingDayOfWeek)
Iteration #3
--iteration 3
SELECT DATEADD(DAY,DATEDIFF(DAY,0,#Currentdate)/7*7 +
CASE WHEN (DATEPART(weekday,#Startdate)-2) < (DATEPART(weekday,#Currentdate)-2)
then 7
ELSE 0
end
,(DATEPART(weekday,#Startdate)-2))
Hat tip to this article:
http://www.sqlmag.com/article/tsql3/datetime-calculations-part-3

Answer 2

Here's what I came up with thanks to TetonSig and his reference to this link: http://www.sqlmag.com/article/tsql3/datetime-calculations-part-3
We can get the date of the previous Monday exclusive of the current date (#AsOfDate) like so:
SELECT DATEADD(day, DATEDIFF(day,0, #AsOfDate-1) /7*7, 0);
This gets the number of days between 1/1/1900 and #AsOfDate in days. /7*7 converts that to whole weeks, and then adds it back to 1/1/1900 (a Mon) to get the Monday before #AsOfDate. The -1 makes it exclusive of #AsOfDate. Without the minus 1, if #AsOfDate were on a Monday, it would be counted as the "previous" Monday.
Next the author shows that to get the inclusive next Monday, we simply need to add 7 to the exclusive previous Monday formula:
SELECT DATEADD(d, DATEDIFF(day,0, #AsOfDate-1) /7*7, 0)+7;
Voila! We've now got the first Monday on or after #AsOfDate. The only problem is, the Monday (0) above is a moving target in my case. I need the first [DayOfWeek] determined by the class date, not the first Monday. I need to swap out a ClassDayOfWeek calculation for the 0s above:
DATEADD(d, DATEDIFF(d, [ClassDayOfWeek], #AsOfDate-1)/7*7, [ClassDayOfWeek])+7
I wanted to calculate the ClassDayOfWeek without being dependent on or having to mess with setting ##datefirst. So I calculated it relative to the base date:
DATEDIFF(d, 0, StartDate)%7
This gives 0 for Mon, 6 for Sun so we can now plug that in for [ClassDayOfWeek]. I should point out that this 0-6 value is dates 1/1/1900-1/7/1900 represented as an int.
DATEADD(d, DATEDIFF(d, DATEDIFF(d, 0, StartDate)%7, #AsOfDate-1)/7*7, DATEDIFF(d, 0, StartDate)%7)+7
And in use per the question:
SELECT *
FROM tbl_Class cs
INNER JOIN tbl_Enrollment sce ON cs.pk_ClassID = sce.fk_ClassID
WHERE ...
AND sce.StartDate < DATEADD(d,
DATEDIFF(d,
DATEDIFF(d, 0, cs.StartDate)%7,
#AsOfDate-1)/7*7,
DATEDIFF(d, 0, cs.StartDate)%7)+7

Answer 3

I derived the answer with a simple case statement.
In your situation #targetDOW would be the day of the week of the class.
DECLARE #todayDOW INT = DATEPART(dw, GETDATE());
DECLARE #diff INT = (#targetDOW - #todayDOW);
SELECT
CASE
WHEN #diff = 0 THEN GETDATE()
WHEN #diff > 0 THEN DATEADD(d,#diff,GETDATE())
WHEN #diff < 0 THEN DATEADD(d,#diff + 7,GETDATE())
END;