using round and is null

select od.OrderID,p.CategoryID , year(o.OrderDate) year, month(o.OrderDate) month,
round(
    od.UnitPrice * od.Quantity*
    isnull(
        (select top 1 rate from ExchangeRate er 
        where er.Currency = 'USD' and er.EffectiveDate <=o.OrderDate
        order by er.EffectiveDate desc)
    ,1)
,2)
rowTotal

from [Order Details] od 
	left outer join Products p on p.ProductID = od.ProductID
	left outer join Orders o on o.OrderID = od.OrderID 

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