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

Join option 1 is the equivalent of:

SELECT *
FROM   TXN
       LEFT OUTER JOIN CHK
       ON ( TXN.CHK_ID = CHK.CHK_ID )
WHERE  TXN.CURRENT = 'Y'
AND    CHK.CURRENT = 'Y'

However, since any rows that are in the result set will have CHK.CURRENT = 'Y' (a non-null value) then those rows must have matched on CHK_ID and the query is actually the equivalent of:

SELECT *
FROM   TXN
       INNER JOIN CHK
       ON ( TXN.CHK_ID = CHK.CHK_ID )
WHERE  TXN.CURRENT = 'Y'
AND    CHK.CURRENT = 'Y'

Join option 2 is the equivalent of:

SELECT *
FROM   TXN
       LEFT OUTER JOIN CHK
       ON (    TXN.CHK_ID  = CHK.CHK_ID
           AND CHK.CURRENT = 'Y' )
WHERE  TXN.CURRENT = 'Y'

You can make Join option 1 the equivalent of option 2 using:

SELECT *
FROM   TXN
       LEFT OUTER JOIN CHK
       ON ( TXN.CHK_ID = CHK.CHK_ID )
WHERE  TXN.CURRENT = 'Y'
AND    ( CHK.CURRENT = 'Y' OR CHK.CHK_ID IS NULL )