获取每月第n周第n天的日期

Question

I have a column with values like '3rd-Wednesday', '2nd-Tuesday', 'Every-Thursday'.

I'd like to create a column that reads those strings, and determines if that date has already come this month, and if it has, then return that date of next month. If it has not passed yet for this month, then it would return the date for this month.

Expected results (on 4/22/16) from the above would be: '05-18-2016', '05-10-2016', '04-28-2016'.

I'd prefer to do it mathematically and avoid creating a calendar table if possible.

Thanks.

Answer 1

Partial answer, which is by no means bug free.

This doesn't cater for 'Every-' entries, but hopefully will give you some inspiration. I'm sure there are plenty of test cases this will fail on, and you might be better off writing a stored proc.

I did try to do this by calculating the day name and day number of the first day of the month, then calculating the next wanted day and applying an offset, but it got messy. I know you said no date table but the CTE simplifies things.

How it works

A CTE creates a calendar for the current month of date and dayname. Some rather suspect parsing code pulls the day name from the test data and joins to the CTE. The where clause filters to dates greater than the Nth occurrence, and the select adds 4 weeks if the date has passed. Or at least that's the theory :)

I'm using DATEFROMPARTS to simplify the code, which is a SQL 2012 function - there are alternatives on SO for 2008.

SELECT * INTO #TEST FROM (VALUES ('3rd-Wednesday'), ('2nd-Tuesday'), ('4th-Monday')) A(Value)

SET DATEFIRST 1

;WITH DAYS AS (
    SELECT 
    CAST(DATEADD(MONTH,DATEDIFF(MONTH,0,GETDATE()),N.Number) AS DATE) Date, 
    DATENAME(WEEKDAY, DATEADD(MONTH,DATEDIFF(MONTH,0,GETDATE()),N.Number)) DayName 
    FROM master..spt_values N WHERE N.type = 'P' AND N.number BETWEEN 0 AND 31
)
SELECT 
    T.Value,
    CASE WHEN MIN(D.Date) < GETDATE() THEN DATEADD(WEEK, 4, MIN(D.DATE)) ELSE MIN(D.DATE) END Date
FROM #TEST T
JOIN DAYS D ON REVERSE(SUBSTRING(REVERSE(T.VALUE), 1, CHARINDEX('-', REVERSE(T.VALUE)) -1)) = D.DayName
WHERE D.Date >=
    DATEFROMPARTS(
        YEAR(GETDATE()), 
        MONTH(GETDATE()),
        1+ 7*(CAST(SUBSTRING(T.Value, 1,1) AS INT) -1)
        )
GROUP BY T.Value



Value         Date
------------- ----------
2nd-Tuesday   2016-05-10
3rd-Wednesday 2016-05-18
4th-Monday    2016-04-25