Alternative to union in Oracle 10g

Question

Below is the sql using union that I am using currently. There are 64 columns from 3 different tables. Out of all the columns only one column value is a constant to get 2 records.
select col1, col2, col3,...., 'DE' medium_val
from table1, table2, table3
where table1.col1 = table2.col1
and table2.col1 = table3.col1
and other criteria
union
select col1, col2, col3,...., 'WG' medium_val
from table1, table2, table3
where table1.col1 = table2.col1
and table2.col1 = table3.col1
and other criteria
Basically just repeating the same code just for one column value. Is there any other alternative to union to avoid querying the same tables twice.

Answer 1

Hi,
Here's one way that will work in Oracle 9 (or higher):
WITH     all_medium_vals     AS
     SELECT  'DE' AS medium_val     FROM dual     UNION ALL
     SELECT     'WG'                FROM dual
SELECT  col1, col2, col3, ....
,      m.medium_val
FROM      table1
,      table2
,      table3
,     all_medium_vals  m
WHERE      table1.col1      = table2.col1
AND     table2.col1      = table3.col1
AND     ...
;Don't reference m.medium_val in the WHERE clause. By leaving it out, you'll do a cross-join of your result set from table1, table2 and table3 with the 2 rows in all_medium_vals.
Starting in Oracle 11, there's also SELECT ... UNPIVOT.
You'll notice this query is still doing a UNION, but it's just doing the UNION of two rows, and then doing the complicated query only once. That will be a lot more efficient than doing the complicated query twice, and then doing a UNION of however many rows that happend to produce. There are ways to generate all_medium_vals without a UNION, but they are not as easy to understand or debug, and not significantly more efficient.