Create a value adding two columns together - sql

Question

I am attempting to define and new column called End Year that is the calculation of another column plus a number representing number of years. For some reason my script does not recognize the new column called Allocation Year using the excerpt below SQL statement in SQL Server 2008. Note that Contract Year is an existing column that is identified in the non-redacted full script:
,[Allocation Type] =
CASE
WHEN left([contract type] ,1)'1' = THEN 'O&M'
ELSE 'N/A'
END
,[Allocation Year] =
CASE
WHEN [Contract Year]='XXXX' THEN '0'
ELSE CAST ([Contract Year] AS INT)
END
,[End Year] =
CASE
WHEN [Allocation Type]='O&M' THEN [Allocation Year] + 6
END

Answer 1

Columns are named with the AS keyword (which can also be elided in many cases), not with the = operator.
SQL Server does not let you reference other columns or aliases in a SELECT clause - you have to use a subquery:
Like so:
SELECT
*,
CASE WHEN [Allocation Type] = 'O&M'
THEN [Allocation Year Temp] + 6
ELSE NULL
END AS [Allocation Year]
FROM
(
SELECT
CASE WHEN LEFT( [Contract type], 1 ) = '1'
THEN 'O&M'
ELSE 'N/A'
END AS [Allocation Type],
CASE WHEN [Contract Year] = 'XXXX'
THEN '0'
ELSE CAST( [Contract Year] AS int )
END AS [Allocation Year Temp],
*
FROM
....
)

Answer 2

If you want to put everything in one select query, then try this one. Hope this helps. Thanks.
,[End Year] =
CASE
WHEN left([contract type] ,1)='1' THEN
(CASE
WHEN [Contract Year]='XXXX' THEN 6
ELSE (CAST ([Contract Year] AS INT) + 6 )
END)
END

Answer 3

You cannot reference computed columns in the SELECT statement. You'll have to repeat the calculation (or use subqueries like Dai suggested):
,[Allocation Type] =
CASE
WHEN left([contract type] ,1)='1' THEN 'O&M'
ELSE 'N/A'
END
,[Allocation Year] =
CASE
WHEN [Contract Year]='XXXX' THEN 0
ELSE CAST ([Contract Year] AS INT)
END
,[End Year] =
CASE
WHEN left([contract type] ,1)='1' THEN (CASE
WHEN [Contract Year]='XXXX' THEN 0
ELSE CAST ([Contract Year] AS INT)
END) + 6
END