Add business days to date in SQL without loops

This answers is based on @ElmerMiller's answer.

It fixes the negative value on Sunday comment from @FistOfFury

Negative values don't work if the date passed in is Sunday

And the DATEFIRST setting comment from @Damien_The_Unbeliever

But this one does assume a particular DATEFIRST setting (7), which some of the others don't need.

Now the corrected function

CREATE FUNCTION[dbo].[AddBusinessDays](@Date DATE,@n INT)
RETURNS DATE AS 
BEGIN
DECLARE @d INT,@f INT,@DW INT;
SET @f=CAST(abs(1^SIGN(DATEPART(DW, @Date)-(7-@@DATEFIRST))) AS BIT)
SET @DW=DATEPART(DW,@Date)-(7-@@DATEFIRST)*(@f^1)+@@DATEFIRST*(@f&1)
SET @d=4-SIGN(@n)*(4-@DW);
RETURN DATEADD(D,@n+((ABS(@n)+(@d%(8+SIGN(@n)))-2)/5)*2*SIGN(@n)-@d/7,@Date);
END

This answer has been significantly altered since it was accepted, since the original was wrong. I'm more confident in the new query though, and it doesn't depend on DATEFIRST

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I think this should cover it:

declare @fromDate datetime
declare @daysToAdd int

select @fromDate = '20130123',@DaysToAdd = 4

declare @Saturday int
select @Saturday = DATEPART(weekday,'20130126')

;with Numbers as (
    select 0 as n union all select 1 union all select 2 union all select 3 union all select 4
), Split as (
    select @DaysToAdd%5 as PartialDays,@DaysToAdd/5 as WeeksToAdd
), WeekendCheck as (
    select WeeksToAdd,PartialDays,MAX(CASE WHEN DATEPART(weekday,DATEADD(day,n.n,@fromDate))=@Saturday THEN 1 ELSE 0 END) as HitWeekend
    from
    Split t
        left join
    Numbers n
        on
            t.PartialDays >= n.n
group by WeeksToAdd,PartialDays
)
select DATEADD(day,WeeksToAdd*7+PartialDays+CASE WHEN HitWeekend=1 THEN 2 ELSE 0 END,@fromDate)
from WeekendCheck

We split the time to be added into a number of weeks and a number of days within a week. We then use a small numbers table to work out if adding those few days will result in us hitting a Saturday. If it does, then we need to add 2 more days onto the total.

This is better if anyone is looking for a TSQL solution. No loops, no tables, no case statements AND works with negatives. Can anyone beat that?

CREATE FUNCTION[dbo].[AddBusinessDays](@Date date,@n INT)
RETURNS DATE AS 
BEGIN
DECLARE @d INT;SET @d=4-SIGN(@n)*(4-DATEPART(DW,@Date));
RETURN DATEADD(D,@n+((ABS(@n)+@d-2)/5)*2*SIGN(@n)-@d/7,@Date);
END