Question

I have a query like this which i am supposed to refactor:

SELECT *
  FROM a a1
      ,b common_alias
      ,b common_alias1
 WHERE a1.id = common_alias.id(+)
   AND a1.id = common_alias1.id(+)
   AND common_alias.name = 'XYZ'
   AND common_alias1.name = 'XYZ'
UNION
SELECT *
  FROM a a1
      ,b common_alias
      ,b common_alias1
 WHERE a1.id = common_alias.id(+)
   AND a1.id = common_alias1.id(+)
   AND common_alias.name = 'XYZ'
   AND common_alias1.name = 'PQR'

It seems to me that i can remove the first query completely. Is there any case where removing the first query will make a difference?

Answer 1

Your outer join condition on common_alias1.id should be reviewed, it has no effect, as you don’t outer join on common_alias1.name as well. See Oracle outer join with filter condition on the second table.

But back to your question: no, you can’t remove the first part, where would you get the XYZ|XYZ row? See http://sqlfiddle.com/#!4/c7dfa/12 for an example.

But you can refactor to

SELECT a1.*,common_alias.name n1,common_alias1.name n2
  FROM a a1
      ,b common_alias
      ,b common_alias1
 WHERE a1.id = common_alias.id(+)
   AND a1.id = common_alias1.id(+)
   AND common_alias.name = 'XYZ'
   AND (common_alias1.name = 'XYZ' OR common_alias1.name = 'PQR')
;