Use value of a column for another column (SQL Server)?

Question

lets say I have a huge select on a certain table. One value for a column is calculated with complex logc and its called ColumnA. Now, for another column, I need the value from ColumnA and add some other static value to it.

Sample SQL:

select table.id, table.number, complex stuff [ColumnA], [ColumnA] + 10 .. from table ...

The [ColumnA] + 10 is what im looking for. The complex stuff is a huge case/when block.

Ideas?

Answer 1

If you want to reference a value that's computed in the SELECT clause, you need to move the existing query into a sub-SELECT:

SELECT
    /* Other columns */,
    ColumnA,
    ColumnA + 10 as ColumnB
FROM
(select table.id, table.number, complex stuff [ColumnA].. from table ...
) t

You have to introduce an alias for this table (in the above, t, after the closing bracket) even if you're not going to use it.

(Equivalently - assuming you're using SQL Server 2005 or later - you can move your existing query into a CTE):

;WITH PartialResults as (
     select table.id, table.number, complex stuff [ColumnA].. from table ...
)
SELECT /* other columns */, ColumnA, ColumnA+10 as ColumnB from PartialResults

CTEs tend to look cleaner if you've got multiple levels of partial computations being done, I.e. if you've now got a calculation that depends on ColumnB to include in your query.