Oracle outer join strange results (at least for me)

Question

Hi there,
I want to show you a problem I saw to a client site.
Oracle 9.2.0.8.0
SunOS 5.9 (but I'm unable to check installed patch)
The query is quite simple:
SELECT *
  FROM     (SELECT /*+ NO_MERGE */
                  DISTINCT col1,col2
              FROM Tab1
             WHERE status = 'L') a
       LEFT JOIN
           (SELECT col1,
                   col2,
                   1 as flag_match
              FROM tab2 a
             WHERE     a.action IN ('I', 'U')
                   AND NVL (a.col3, 'D') IN ('I', 'U')
                   AND NVL (a.col4, '?') NOT IN ('V', 'P', 'Q')) b
       ON (b.col1 = a.col1 AND b.col2 = a.col2)The expected result is (obvious?):
a.Col1   a.Col2   b.Col1   b.Col2   flag_match
X          Y         NULL    NULL     NULL
A          B         A         B          1What we get is:
a.Col1   a.Col2   b.Col1   b.Col2   flag_match
X          Y         NULL    NULL     NULL
A          B         A         B          NULLThis looks wrong to me.
If we check with a SELECT COUNT(*) the result of the query, using something like this:
SELECT COUINT(*) FROM (
SELECT *
  FROM     (SELECT /*+ NO_MERGE */
                  DISTINCT col1,col2
              FROM Tab1
             WHERE status = 'L') a
       LEFT JOIN
           (SELECT col1,
                   col2,
                   1 as flag_match
              FROM tab2 a
             WHERE     a.action IN ('I', 'U')
                   AND NVL (a.col3, 'D') IN ('I', 'U')
                   AND NVL (a.col4, '?') NOT IN ('V', 'P', 'Q')) b
       ON (b.col1 = a.col1 AND b.col2 = a.col2)
WHERE flag_match...We got these results:
with FLAG_MATCH IS NULL, 1000 rows
with FLAG_MATCH IS NOT NULL, 900 rows
without the where on FLAG_MATCH, 1000 rows
What you think? Ever seen something like this?
Thank you in advance.
Antonio

Answer 1

Hi Antonio,
running the query as you have posted, you should not get the result NULL under column flag_match when b.col1 is not NULL.
Unless a bug but I doubt it.
Could you please post some sample data to be able to reproduce the problem?
Data can be posted also as WITH statement in this way:
WITH tab1(col1, col2, status) AS
   SELECT 'X', 'Y', 'L' FROM DUAL UNION ALL
   SELECT 'A', 'B', 'L' FROM DUAL
, tab2(col1, col2, col3, col4, action) as
   SELECT 'A', 'B', 'I', 'X', 'I' FROM DUAL
SELECT *
  FROM     (SELECT /*+ NO_MERGE */
                  DISTINCT col1,col2
              FROM Tab1
             WHERE status = 'L') a
       LEFT JOIN
           (SELECT col1,
                   col2,
                   1 as flag_match
              FROM tab2 a
             WHERE     a.action IN ('I', 'U')
                   AND NVL (a.col3, 'D') IN ('I', 'U')
                   AND NVL (a.col4, '?') NOT IN ('V', 'P', 'Q')) b
       ON (b.col1 = a.col1 AND b.col2 = a.col2)
COL1 COL2 COL1_1 COL2_1 FLAG_MATCH
A    B    A      B               1
X    Y                            Also try to execute the statement above to see if you are experiencing the same problem.
Edit:
I've seen the second part of your post:
with FLAG_MATCH IS NULL, 1000 rows
with FLAG_MATCH IS NOT NULL, 900 rows
without the where on FLAG_MATCH, 1000 rowsand I have to say that is looks like a bug. Could you try grouping by flag_match?
SELECT flag_match, COUNT(*) FROM (
SELECT *
  FROM     (SELECT /*+ NO_MERGE */
                  DISTINCT col1,col2
              FROM Tab1
             WHERE status = 'L') a
       LEFT JOIN
           (SELECT col1,
                   col2,
                   1 as flag_match
              FROM tab2 a
             WHERE     a.action IN ('I', 'U')
                   AND NVL (a.col3, 'D') IN ('I', 'U')
                   AND NVL (a.col4, '?') NOT IN ('V', 'P', 'Q')) b
       ON (b.col1 = a.col1 AND b.col2 = a.col2)
GROUP by flag_matchRegards.
Al
Edited by: Alberto Faenza on Dec 18, 2012 2:49 PM