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Declare @aDate datetime,
@bDate datetime
SELECT @aDate = '9/7/2010', @bDate = '9/13/2010'

SELECT datediff(dy,@aDate,@bDate) - (datediff(dy,@aDate,@bDate)/7*2 +
case when datepart(dw,@aDate)=1 and datediff(dy,@aDate,@bDate)%7<6 then 1
when datepart(dw,@aDate)=1 and datediff(dy,@aDate,@bDate)%7=6 then 2
when datepart(dw,@aDate)>1 and datepart(dw,@aDate)+datediff(dy,@aDate,@bDate)=7 then 1
when datepart(dw,@aDate)>1 and datepart(dw,@aDate)+datediff(dy,@aDate,@bDate)>7 then 2
else 0 end)