Problem using alias field names in a sql query

Question

Hello,
I have a question regarding a simple Oracle database SQL query writeup:
In the following (badly written but properly working) SQL query:
Query 1:
select
soe.field1,
(soe.field2 + soe.field3) as field4,
(soe.field5 - (soe.field2 + soe.field3)) as field6,
(select comp.parValue*soe.field7
from
CompTable comp) as parValue,
(select soe.field8 - (comp.parValue*soe.field7+ soe.field9)
from
CompTable comp) as field10
from
SomeTable soe
PROBLEM 1:
I am re writing the code (soe.field2 + soe.field3) to get the alias field4 or column name field4 that I have created on the fly in the previously for use with the following fields. Can't I rewrite the query as follows. There is something simple missing!
Query 2:
select
soe.field1,
(soe.field2 + soe.field3) as field4,
soe.field5 - field4 as field6, <<< field4 does not work here
(select
comp.parValue*soe.field7
from
CompTable comp) as parValue,
(select
soe.field8 - (comp.parValue*soe.field7+ soe.field9)
from
CompTable comp) as field10
from
SomeTable soe
PROBLEM 2:
Similar to the above problem, I was thinking to get a field parValue out of the CompTable table and re-use many times rather than the code shown in Query 1:
Query 3:
select
soe.field1,
(soe.field2 + soe.field3) as field4,
soe.field5 - field4 as field6,
soe.field7* (select comp.parValue from CompTable comp) as parValue1,
soe.field8 - (parValue1*soe.field7+ soe.field9) as field10      <<<< parvalue1 does not work here
parValue1*soe.field9 as TaxCondition               <<<< parvalue1 does not work here
from
SomeTable soe
See that the query becomes so simple, but the above query does not work. There is something fundamentally wrong in my usage of the alias field names in creating other fields. The Query1 seems to be the only working option but its very slow as I am redoing and re-writing the whole code again and again to get the parValue field out of the CompTable table for use to create many other fields.
I will appreciate if you can guide me in the right direction on this issue.
Thanks and Regards
Rama

Answer 1

SELECT tmp.contract_no, tmp.Actual, tmp.Actual - tmp.NbHours
FROM ( SELECT t.contract_no, sum(l.hrs) AS Actual, (c.labour_hours * c.labour_progress_per) / 100 AS NbHours
FROM TASK_DELEGATION t
INNER JOIN COST_CODE c
ON t.cost_code = c.cost_code AND t.contract_no = c.contract_no AND t.is_inactive=0
INNER JOIN Labour.dbo.LABOURALLOT l
ON l.contractNo = c.contract_no AND l.costcode = c.cost_code AND l.pm = 'N'
GROUP BY t.contract_no, c.labour_hours, c.labour_progress_per
) tmp