Oracle (+) outer join and constant values

Question

I\'m running into an issue with which I can\'t figure out how to correctly configure a join. I\'m using reporting software that utilizes the (+) indicators in the WHERE clause f

Answer 1

.The (+) operator tells Oracle that a predicate is part of an outer join rather than a filter predicate that can be applied after the join. Using the SQL 99 outer join syntax, the first query is equivalent to

SELECT *
  FROM txn
       left outer join chk 
         on( txn.chk_id = chk.chk_id )
 WHERE txn.current = 'Y'
   AND chk.current = 'Y'

while the second is equivalent to

SELECT *
  FROM txn
       left outer join chk 
         on( txn.chk_id  = chk.chk_id AND
             chk.current = 'Y')
 WHERE txn.current = 'Y'

Logically, in the first case, you do the outer join but then all the rows where chk.current was NULL get filtered out. In the second case, the chk.current = 'Y' condition doesn't filter out any rows, it just controls whether a matching row is found in chk or whether a left outer join is performed.