Question

Given a start and end time, I want to know how many minutes in a given hour are covered.

create function CalcMinsInHour(@start datetime, @end datetime, @hour int)
returns int
as
begin
    --Looking for best way to write this part
end

CalcMinsInHour('2012-01-18 8:15', '2012-01-18 10:30', 7)  should return 0
CalcMinsInHour('2012-01-18 8:15', '2012-01-18 10:30', 8)  should return 45
CalcMinsInHour('2012-01-18 8:15', '2012-01-18 10:30', 9)  should return 60
CalcMinsInHour('2012-01-18 8:15', '2012-01-18 10:30', 10) should return 30
CalcMinsInHour('2012-01-18 8:15', '2012-01-18 10:30', 11) should return 0

Edit: @Start and @End represent employee clock in/out times. So yes they can span two days when they work past midnight, but not more than that.

Answer 1

This should do the trick:

ALTER FUNCTION dbo.CalcMinsInHour(@start DATETIME, @end DATETIME, @hour INTEGER)
RETURNS INTEGER 
AS
BEGIN
    DECLARE @StartOfHour DATETIME
    DECLARE @EndOfHour DATETIME
    SELECT @StartOfHour = DATEADD(hh, @hour, CAST(CAST(@start AS DATE) AS DATETIME))
    IF NOT (@StartOfHour BETWEEN @start and @end)
        SET @StartOfHour = DATEADD(hh, @hour, CAST(CAST(@end AS DATE) AS DATETIME))

    SELECT @EndOfHour = DATEADD(hh, 1, @StartOfHour)

RETURN
(
SELECT 
    CASE WHEN @EndOfHour < @start OR @StartOfHour > @end THEN 0 ELSE
        DATEDIFF(mi,
            CASE WHEN @StartOfHour <= @start THEN @start ELSE @StartOfHour END,
            CASE WHEN @EndOfHour > @end THEN @end ELSE @EndOfHour END)  
    END
)
END