SQL Union of columns from same subquery into a single one

Question

I want to select a single column given as the union of the content in cols 1, 2, 3, 4 of the same table. Since SUBQUERY and CONDITION are complicated I have the feeling that this is the wrong way to go for it:

SELECT COL1 FROM SUBQUERY WHERE CONDITION 
UNION
SELECT COL2 FROM SUBQUERY WHERE CONDITION 
UNION
SELECT COL3 FROM SUBQUERY WHERE CONDITION 
UNION
SELECT COL4 FROM SUBQUERY WHERE CONDITION 
UNION
SELECT COL5 FROM SUBQUERY WHERE CONDITION 

Since this query looks bad both performance-wise and from the point of view of good coding style, Is there a better syntax in Oracle?

P.S.

If only the following pseudo code were correct... it would be perfect:

SELECT COL1 UNION COL2 UNION COL3 UNION COL4 
FROM 
    SUBQUERY WHERE CONDITION;

Answer 1

You can use UNPIVOT:

select * from 
    (select 'a' col1, 'b' col2, 'c' col3, 'd' col4, 'e' col5 from dual)
unpivot (
  united_columns for subquery_column in ("COL1", "COL2", "COL3", "COL4", "COL5"));

SUBQUERY_COLUMN UNITED_COLUMNS
--------------- --------------
COL1            a              
COL2            b              
COL3            c              
COL4            d              
COL5            e          

Place your select * from subquery where condition istead of

select 'a' col1, 'b' col2, 'c' col3, 'd' col4, 'e' col5 from dual

More information about UNPIVOT to get the idea: http://www.oracle.com/technetwork/articles/sql/11g-pivot-097235.html