SQL Nth Day of Nth Week of a Month - sql

Question

I want to use the following function for scheduling club meetings which occur on a monthly basis based on the week and weekday of the month. In the example below I have a (to be) function that returns the Third Wednesday of the month. If that day occurs in the past then it returns the next month's 3rd Wednesday.
I want to get away from the Loops and I feel that there is a better method for calculation. Is there a more OO process? Your opinion?
--CREATE FUNCTION NextWeekDayofMonth
DECLARE
--(
#WEEK INT,
#WEEKDAY INT,
#REFERENCEDATE DATETIME
--)
--RETURNS DATETIME
--AS
-------------------------------
--Values for testing - Third Wednesday of the Month
set #WEEK = 3 --Third Week
set #WEEKDAY = 4 --Wednesday
set #REFERENCEDATE = '08/20/2011'
-------------------------------
BEGIN
DECLARE #WEEKSEARCH INT
DECLARE #FDOM DATETIME
DECLARE #RETURNDATE DATETIME
SET #FDOM = DATEADD(M,DATEDIFF(M,0,#REFERENCEDATE),0)
SET #RETURNDATE = DATEADD(M,0,#FDOM)
WHILE (#RETURNDATE < #REFERENCEDATE)
--If the calculated date occurs in the past then it
--finds the appropriate date in the next month
BEGIN
SET #WEEKSEARCH = 1
SET #RETURNDATE = #FDOM
--Finds the first weekday of the month that matches the provided weekday value
WHILE ( DATEPART(DW,#RETURNDATE) <> #WEEKDAY)
BEGIN
SET #RETURNDATE = DATEADD(D,1,#RETURNDATE)
END
--Iterates through the weeks without going into next month
WHILE #WEEKSEARCH < #WEEK
BEGIN
IF MONTH(DATEADD(WK,1,#RETURNDATE)) = MONTH(#FDOM)
BEGIN
SET #RETURNDATE = DATEADD(WK,1,#RETURNDATE)
SET #WEEKSEARCH = #WEEKSEARCH+1
END
ELSE
BREAK
END
SET #FDOM = DATEADD(M,1,#FDOM)
END
--RETURN #RETURNDATE
select #ReturnDate
END

Answer 1

IMO, the best process is to store important business information as rows in tables in a database. If you build a calendar table, you can get all the third Wednesdays by a simple query. Not only are the queries simple, they can be seen to be obviously correct.
select cal_date
from calendar
where day_of_week_ordinal = 3
and day_of_week = 'Wed';
The third Wednesday that's on or after today is also simple.
select min(cal_date)
from calendar
where day_of_week_ordinal = 3
and day_of_week = 'Wed'
and cal_date >= CURRENT_DATE;
Creating a calendar table is straightforward. This was written for PostgreSQL, but it's entirely standard SQL (I think) except for the columns relating to ISO years and ISO weeks.
create table calendar (
cal_date date primary key,
year_of_date integer not null
check (year_of_date = extract(year from cal_date)),
month_of_year integer not null
check (month_of_year = extract(month from cal_date)),
day_of_month integer not null
check (day_of_month = extract(day from cal_date)),
day_of_week char(3) not null
check (day_of_week =
case when extract(dow from cal_date) = 0 then 'Sun'
when extract(dow from cal_date) = 1 then 'Mon'
when extract(dow from cal_date) = 2 then 'Tue'
when extract(dow from cal_date) = 3 then 'Wed'
when extract(dow from cal_date) = 4 then 'Thu'
when extract(dow from cal_date) = 5 then 'Fri'
when extract(dow from cal_date) = 6 then 'Sat'
end),
day_of_week_ordinal integer not null
check (day_of_week_ordinal =
case
when day_of_month >= 1 and day_of_month <= 7 then 1
when day_of_month >= 8 and day_of_month <= 14 then 2
when day_of_month >= 15 and day_of_month <= 21 then 3
when day_of_month >= 22 and day_of_month <= 28 then 4
else 5
end),
iso_year integer not null
check (iso_year = extract(isoyear from cal_date)),
iso_week integer not null
check (iso_week = extract(week from cal_date))
);
You can populate that table with a spreadsheet or with a UDF. Spreadsheets usually have pretty good date and time functions. I have a UDF, but it's written for PostgreSQL (PL/PGSQL), so I'm not sure how much it would help you. But I'll post it later if you like.

Answer 2

Here's a date math way to accomplish what you want without looping:
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
-- =============================================
-- Description: Gets the nth occurrence of a given weekday in the month containing the specified date.
-- For #dayOfWeek, 1 = Sunday, 2 = Monday, 3 = Tuesday, 4 = Wednesday, 5 = Thursday, 6 = Friday, 7 = Saturday
-- =============================================
CREATE FUNCTION GetWeekdayInMonth
(
#date datetime,
#dayOfWeek int,
#nthWeekdayInMonth int
)
RETURNS datetime
AS
BEGIN
DECLARE #beginMonth datetime
DECLARE #offSet int
DECLARE #firstWeekdayOfMonth datetime
DECLARE #result datetime
SET #beginMonth = DATEADD(DAY, -DATEPART(DAY, #date) + 1, #date)
SET #offSet = #dayOfWeek - DATEPART(dw, #beginMonth)
IF (#offSet < 0)
BEGIN
SET #firstWeekdayOfMonth = DATEADD(d, 7 + #offSet, #beginMonth)
END
ELSE
BEGIN
SET #firstWeekdayOfMonth = DATEADD(d, #offSet, #beginMonth)
END
SET #result = DATEADD(WEEK, #nthWeekdayInMonth - 1, #firstWeekdayOfMonth)
IF (NOT(MONTH(#beginMonth) = MONTH(#result)))
BEGIN
SET #result = NULL
END
RETURN #result
END
GO
DECLARE #nextMeetingDate datetime
SET #nextMeetingDate = dbo.GetWeekdayInMonth(GETDATE(), 4, 3)
IF (#nextMeetingDate IS NULL OR #nextMeetingDate < GETDATE())
BEGIN
SET #nextMeetingDate = dbo.GetWeekDayInMonth(DATEADD(MONTH, 1, GETDATE()), 4, 3)
END
SELECT #nextMeetingDate

Answer 3

Here's another function based solution using date math that returns the Next Nth Weekday on or after a given date. It doesn't use any looping to speak of, but it may recurs by at most one iteration if the next Nth weekday is in the next month.
This function takes the DATEFIRST setting into account for environments that use a value other than the default.
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
-- =============================================
-- Author: David Grimberg
-- Create date: 2015-06-18
-- Description: Gets the next Nth weekday
-- #param Date is any date in a month
-- #param DayOfWeek is the weekday of interest ranging
-- from 1 to 7 with ##DATEFIRST being the
-- first day of the week
-- #param NthWeekday represents which ordinal weekday to return.
-- Positive values return dates relative to the start
-- of the month. Negative values return dates relative
-- to the end of the month. Values > 4 indicate the
-- last week, values < -4 indicate the first week.
-- Zero is assumed to be 1.
-- =============================================
ALTER FUNCTION dbo.xxGetNextNthWeekday
(
#Date date,
#NthWeekday smallint,
#DayOfWeek tinyint
)
RETURNS date
AS
BEGIN
DECLARE #FirstOfMonth date
DECLARE #inc int
DECLARE #Result date
-- Clamp the #NthWeekday input to valid values
set #NthWeekday = case when #NthWeekday = 0 then 1
when #NthWeekday > 4 then -1
when #NthWeekday < -4 then 1
else #NthWeekday
end
-- Normalize the requested day of week taking
-- ##DATEFIRST into consideration.
set #DayOfWeek = (##DATEFIRST + 6 + #DayOfWeek) % 7 + 1
-- Gets the first of the current month or the
-- next month if #NthWeekday is negative.
set #FirstOfMonth = dateadd(month, datediff(month,0,#Date)
+ case when #NthWeekday < 0 then 1 else 0 end
, 0)
-- Add and/or subtract 1 week depending direction of search and the
-- relationship of #FirstOfMonth's Day of the Week to the #DayOfWeek
-- of interest
set #inc = case when (datepart(WEEKDAY, #FirstOfMonth)+##DATEFIRST-1)%7+1 > #DayOfWeek
then 0
else -1
end
+ case when #NthWeekday < 0 then 1 else 0 end
-- Put it all together
set #Result = dateadd( day
, #DayOfWeek-1
, dateadd( WEEK
, #NthWeekday + #inc
, dateadd( WEEK -- Gets 1st Sunday on or
, datediff(WEEK, -1, #FirstOfMonth)
,-1 ) ) ) -- before #FirstOfMonth
-- [Snip here] --
if #Result < #Date
set #Result = dbo.xxGetNextNthWeekday( dateadd(month, datediff(month, 0, #Date)+1, 0)
, #NthWeekday, #DayOfWeek)
-- [to here for no recursion] --
Return #Result
END
If you want the past or future Nth weekday of the current month based on the #Date parameter rather than the next Nth weekday snip out the recursive part as indicated above.