SQL query computation help

Question

Hi,
I have a query which computes values and assigns alias names to it. If I want to do more computations in the same query using already computed values, how can I achieve it?
For example,
If I have a table with columns A, B ,C,D and it has one row 1,2,3,4
TABLE
A B C D
1 2 3 4
The query looks like this
Select A, (A+B+C+D) Sum_all, (A+B) Sum_three from table;
Now in my select statement If I need one more value say (C+D), How can I accomplish this with already computed values (Sum_all and Sum_three) ?
Thanks in advance.

Answer 1

Hi,
So you want to say
sum_all - sum_threerather than
c + d? Personally, I find the second way easier and clearer, and they aren't necessarily the same if the columns can be NULL, but I think I understand your question.
Aliases assigned in the SELECT clause can't be referenced in the same SELECT clause.
You can get around that by assigning aliases in a sub-query, then referencing them in a super-query, like this:
WITH  sub_q  AS
    Select A, (A+B+C+D) Sum_all, (A+B) Sum_three from table_x
SELECT  sub_q.*
,       sum_all - sum_three  AS sum_cd
FROM    sub_q;