SQL & PL/SQL

Hi All,
I want to use a calculated column of query to create further calculated columns in same query using column alias. I have to do so because the calculation is too big to write it everytime.
e.g.

Select decode(Month, 1, Tran_January, 2, Tran_February, ....) as MonthTran,
Month, Year,
round(MonthTran/Intrest_rate, 2) as Amount <---(using previous alias)
from ledger;

Thanks in advance,
Mona