Count work days between two dates

Question

How can I calculate the number of work days between two dates in SQL Server?

Monday to Friday and it must be T-SQL.

Answer 1

Using a date table:

    DECLARE 
        @StartDate date = '2014-01-01',
        @EndDate date = '2014-01-31'; 
    SELECT 
        COUNT(*) As NumberOfWeekDays
    FROM dbo.Calendar
    WHERE CalendarDate BETWEEN @StartDate AND @EndDate
      AND IsWorkDay = 1;

If you don't have that, you can use a numbers table:

    DECLARE 
    @StartDate datetime = '2014-01-01',
    @EndDate datetime = '2014-01-31'; 
    SELECT 
    SUM(CASE WHEN DATEPART(dw, DATEADD(dd, Number-1, @StartDate)) BETWEEN 2 AND 6 THEN 1 ELSE 0 END) As NumberOfWeekDays
    FROM dbo.Numbers
    WHERE Number <= DATEDIFF(dd, @StartDate, @EndDate) + 1 -- Number table starts at 1, we want a 0 base

They should both be fast and it takes out the ambiguity/complexity. The first option is the best but if you don't have a calendar table you can allways create a numbers table with a CTE.