Question

I would like to know if the following is possible in SQL server 2005. Column A and B are calculated using other case statements in my actual stored proc. I don’t want to repeat the same for another field unnecessarily. If this is not syntactically possible, any other ideas?

SELECT A, B, CASE WHEN column1='1' THEN A ELSE B END Col1.

Modified version of actual query provided as requested. CTE kind of seems to be tough in this model. WANNABE is the column I want to accomplish in the sub select statement.

SELECT 1 AS Region, 'Test', 
        CAST(Work AS NUMERIC(18,2)) Work,
        Work + 2 AS Work2,
        WANNABE

        FROM
        (
            SELECT 
                ROW_NUMBER() OVER(PARTITION BY G.Value, C.C, FR.Mod1 ORDER BY FR.Date DESC, FG.Date DESC, FC.Date DESC) ROW,
                CASE WHEN COALESCE(FR.Mod1, '') = '' THEN '' ELSE FR.Mod1 END Mod1,

                CASE WHEN @var1=1 AND @var2 = 1 THEN FR.Col1 *  G.Value 
                     WHEN @var1=1 AND @var2 = 0 THEN FP.Col1 *  G.Value END Work,

                CASE WHEN 1=1 THEN Work ELSE 1 END WANNABE,

                (
                    SELECT Col3
                    FROM Table2 
                    WHERE c = FR.Value
                ) AS Custom

            FROM MainTable FR
            JOIN @C C ON FR.Col2 = C.Col2
            LEFT JOIN Function1(@VersionDate) cv ON cv.Code = C.Code    
            LEFT JOIN Function2(@VersionDate) hv ON hv.Code = C.Code    
            LEFT JOIN @G G ON 1 = 1
            LEFT JOIN SubTable1 FG ON FG.Number = G.Value, 2 AND FG.Date = @VersionDate
            LEFT JOIN SubTable2 FO ON FO.Number = G.Value
                AND FO.Date = @VersionDate AND FO.Code = FR.Code AND FR.Mod1 = FO.Mod1
            LEFT JOIN SubTable3 FP ON FP.Code = FR.Code AND FP.VersionDate = @Versiondate
             AND CASE WHEN DATALENGTH(FR.Mod1) = 0 THEN '00' ELSE FR.Mod1 END = CASE WHEN DATALENGTH(FP.Mod1) = 0 THEN '00' ELSE FP.Mod1 END
            LEFT JOIN SubTable4 FC ON FC.Date = @VersionDate
            WHERE  FR.Date = @VersionDate 
        ) x
        WHERE x.Row = 1
        AND RTRIM(LTRIM(x.Col1)) IN ('', '2')

Answer 1

You can define the A,B column aliases in a CTE then reference them in an outer select.

;WITH CTE AS
(
SELECT CASE ... END AS A, 
       CASE ... END  AS B, 
       column1
FROM your_table
)
SELECT A, 
       B, 
       CASE WHEN column1='1' THEN A ELSE B END Col1
FROM  CTE 

Similarly you can also define them in a CROSS APPLY that is sometimes a bit less verbose.
A silly example just to show the syntax is

SELECT A,
       B,
       CASE WHEN type='P' THEN A ELSE B END Col1 
FROM master..spt_values
CROSS APPLY (SELECT CASE WHEN number %2 = 1 THEN 1 END, 
                    CASE WHEN number %2 = 0 THEN 0 END) T(A,B)

Following your update you can replace the derived table with a CTE and nest CTEs as follows

;WITH x as
(
                SELECT 
                    ROW_NUMBER() OVER(PARTITION BY G.Value, C.Code, FR.Mod1 ORDER BY FR.Date DESC, FG.Date DESC, FC.Date DESC) ROW,
...<snip>                
                WHERE  FR.Date = @VersionDate 
),
x2 As
(
SELECT *,
       CASE WHEN 1=1 THEN Work ELSE 1 END WANNABE
FROM x
)
    SELECT 1 AS Region, 'Test', 
            CAST(Work AS NUMERIC(18,2)) Work,
            Work + 2 AS Work2,
            WANNABE
            FROM x2
            WHERE x2.Row = 1
            AND RTRIM(LTRIM(x2.Col1)) IN ('', '2')