How to use a calculated column to calculate another column in the same view

Question

I am hoping you can help with this question. I am using Oracle SQL (SQL Developer for this view)...

If I have a table with the following columns:

• ColumnA (Number)
• ColumnB (Number)
• ColumnC (Number)

In my view I have

Select  
ColumnA,
ColumnB,
ColumnA + ColumnB As calccolumn1

Now at this point, I want to use calccolumn1 but I cannot just say...

Select  
ColumnA,
ColumnB,
ColumnA + ColumnB As calccolumn1
calccolumn1 / ColumnC as calccolumn2

I am assuming I need some type of subquery..but this is where I need your help... How would I word the query so that I can use calccolumn1 in another calculation within the same query? It might be an If then or a Case when, but bottomline it is some derived number.

Answer 1

You could use a nested query:

Select
  ColumnA,
  ColumnB,
  calccolumn1,
  calccolumn1 / ColumnC as calccolumn2
From (
  Select
    ColumnA,
    ColumnB,
    ColumnC,
    ColumnA + ColumnB As calccolumn1
  from t42
);

With a row with values 3, 4, 5 that gives:

   COLUMNA    COLUMNB CALCCOLUMN1 CALCCOLUMN2
---------- ---------- ----------- -----------
         3          4           7         1.4

You can also just repeat the first calculation, unless it's really doing something expensive (via a function call, say):

Select
  ColumnA,
  ColumnB,
  ColumnA + ColumnB As calccolumn1,
  (ColumnA + ColumnB) / ColumnC As calccolumn2
from t42; 

   COLUMNA    COLUMNB CALCCOLUMN1 CALCCOLUMN2
---------- ---------- ----------- -----------
         3          4           7         1.4