问如何在多个SELECT和不同条件下使用SQL表别名？

EN

CREATE PROCEDURE [dbo].[Progreereport]
(
@EndDate DATETIME,
@ComparePeriod VARCHAR(10),
@PortfolioId VARCHAR(50) = '6'
)
AS
BEGIN

DECLARE @StartDate DATETIME =
    CASE
        WHEN @ComparePeriod = 'Daily' THEN dbo.DateUtility_MakeBusinessDay(DATEADD(DAY, -1, @EndDate), 'LessThanOrEqualTo')
        WHEN @ComparePeriod = 'Weekly' THEN dbo.DateUtility_GetRelativeDate(@EndDate, 'LessThan', 'End', 'Week', 1)
        WHEN @ComparePeriod = 'MTD' THEN dbo.DateUtility_GetRelativeDate(@EndDate, 'LessThan', 'End', 'Month', 1)
        WHEN @ComparePeriod = 'QTD' THEN dbo.DateUtility_GetRelativeDate(@EndDate, 'LessThan', 'End', 'Quarter', 1)
        WHEN @ComparePeriod = 'YTD' THEN dbo.DateUtility_GetRelativeDate(@EndDate, 'LessThan', 'End', 'Year', 1)
        WHEN @ComparePeriod = 'LTM' THEN DATEADD(MONTH, -12, @EndDate)
        WHEN @ComparePeriod = 'LTD' THEN CAST(Fireball_Configuration.dbo.GetConfigurationValue('Fireball', 'Concerto Credit Opportunity Master Fund I - Inception Date') AS DATETIME)
    END

  DECLARE @PreviousNAV DECIMAL(18,4)
  DECLARE @TodaysNAV DECIMAL(18,4)
  DECLARE @NetChange DECIMAL(18,4)
  DECLARE @BAMLIndex DECIMAL(18,4)

  SELECT ISNULL(@PreviousNAV,0) PreviousNAV 
  FROM Fireball..NAV
  WHERE Date = @StartDate and PortfolioId = @PortfolioId 

  SELECT ISNULL(@TodaysNAV,0) TodaysNAV 
  FROM Fireball..NAV
  WHERE Date = @EndDate and PortfolioId = @PortfolioId 

  SELECT @NetChange = (@TodaysNAV - @PreviousNAV ) 

  SELECT @BAMLIndex  = @NetChange / @TodaysNAV

  END
  GO