Remove subquery from JOIN?

In the following query, I would like the remove the subquery from the JOIN statement (since my two SELECT statements are selecting data from same table). How can I use that alias? Thanks in advance for any help.

declare @StartDate datetime= '8/01/2011'
declare @EndDate datetime = '9/20/2011'
declare @PortfolioId int = 6

SELECT
   Previous.PreviousNAV,
   Todays.TodaysNAV,
   ISNULL((Todays.TodaysNAV-Previous.PreviousNAV),0) NetChange,
   ISNULL((Todays.TodaysNAV-Previous.PreviousNAV) / Todays.TodaysNAV,0) BAMLIndex
FROM
(
   SELECT PortfolioId,ISNULL(NAV,0) PreviousNAV
   FROM Fireball..NAV
   WHERE Date BETWEEN @StartDate AND @EndDate and PortfolioId = @PortfolioId
) Previous
JOIN
(
  开发者_JAVA技巧 SELECT PortfolioId,ISNULL(NAV,0) TodaysNAV
   FROM Fireball..NAV
   WHERE Date = @EndDate and PortfolioId = @PortfolioId
) Todays
ON Previous.PortfolioId = Todays.PortfolioId

---

Just eliminate the subqueries entirely. I don't think they add anything to the query. Try this instead:

SELECT
   Previous.NAV as PreviousNAV,
   Todays.NAV as TodaysNav,
   ISNULL((Todays.NAV-Previous.NAV),0) NetChange,
   ISNULL((Todays.NAV-Previous.NAV) / Todays.NAV,0) BAMLIndex
FROM
    Fireball..NAV as Previous
JOIN
    Fireball..NAV as Todays
    ON Previous.portfolioID = Todays.PortfolioID
WHERE Previous.Date BETWEEN @StartDate AND @EndDate 
    AND Previous.PortfolioId = @PortfolioId
    AND Todays.Date = @EndDate