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

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.

In SQL Server

You can do this using With CTE

WITH common_table_expression (Transact-SQL)

CREATE TABLE tab(ColumnA DECIMAL(10,2), ColumnB DECIMAL(10,2), ColumnC DECIMAL(10,2))

INSERT INTO tab(ColumnA, ColumnB, ColumnC) VALUES (2, 10, 2),(3, 15, 6),(7, 14, 3)

WITH tab_CTE (ColumnA, ColumnB, ColumnC,calccolumn1)  
AS  
(  
Select
    ColumnA,
    ColumnB,
    ColumnC,
    ColumnA + ColumnB As calccolumn1
  from tab
)  

SELECT
  ColumnA,
  ColumnB,
  calccolumn1,
  calccolumn1 / ColumnC AS calccolumn2
FROM  tab_CTE

DBFiddle Demo

In Sql Server

You can do this using cross apply

Select
  ColumnA,
  ColumnB,
  c.calccolumn1 As calccolumn1,
  c.calccolumn1 / ColumnC As calccolumn2
from t42
cross apply (select (ColumnA + ColumnB) as calccolumn1) as c

You have to include the expression for your calculated column:

SELECT  
ColumnA,  
ColumnB,  
ColumnA + ColumnB AS calccolumn1  
(ColumnA + ColumnB) / ColumnC AS calccolumn2

If you want to refer to calculated column on the "same query level" then you could use CROSS APPLY(Oracle 12c):

--Sample data:
CREATE TABLE tab(ColumnA NUMBER(10,2),ColumnB NUMBER(10,2),ColumnC NUMBER(10,2));

INSERT INTO tab(ColumnA, ColumnB, ColumnC) VALUES (2, 10, 2);
INSERT INTO tab(ColumnA, ColumnB, ColumnC) VALUES (3, 15, 6);
INSERT INTO tab(ColumnA, ColumnB, ColumnC) VALUES (7, 14, 3);
COMMIT;

Query:

SELECT
  ColumnA,
  ColumnB,
  sub.calccolumn1,
  sub.calccolumn1 / ColumnC AS calccolumn2
FROM tab t
CROSS APPLY (SELECT t.ColumnA + t.ColumnB AS calccolumn1 FROM dual) sub;

DBFiddle Demo

Please note that expression from CROSS APPLY/OUTER APPLY is available in other clauses too:

SELECT
  ColumnA,
  ColumnB,
  sub.calccolumn1,
  sub.calccolumn1 / ColumnC AS calccolumn2
FROM tab t
CROSS APPLY (SELECT t.ColumnA + t.ColumnB AS calccolumn1 FROM dual) sub
WHERE sub.calccolumn1 = 12;
-- GROUP BY ...
-- ORDER BY ...;

This approach allows to avoid wrapping entire query with outerquery or copy/paste same expression in multiple places(with complex one it could be hard to maintain).

Related article: The SQL Language’s Most Missing Feature