3 tables with left joins - bug?

Question

Hello,
i am making query where i encounter problem with left join in oracle. I am using oracle 10g and i prepare simple test case.
he is testing tables and datas - really simple i think:
drop table t1;
drop table t2;
drop table t3;
create table t1 (a number not null);
create table t2 (a number, b number);
create table t3 (b number);
insert into t3 values (1);
insert into t3 values (2);
insert into t3 values (3);
insert into t1 (a) values (1);
insert into t2 (a,b) values (1,1);
insert into t1 (a) values (2);
insert into t2 (a,b) values (2, null);
insert into t1 (a) values (3);
insert into t1 (a) values (4);
insert into t2 (a,b) values (4,1);
insert into t1 (a) values (5);
insert into t2 (a,b) values (5,3);
and now query with left joins:
select
t1.a
, t2.a, t2.b
, t3.b
from
t1, t2, t3
where
t1.a = t2.a (+)
and t2.b = t3.b (+)
and t3.b is null
order by t1.a
i get two rows as result:
A A_1 B B_1
2 2 null null      
3 null null null                
i expect these rows but when i change my query - i dont want get back t3.b column:
select
t1.a
, t2.a, t2.b
/* , t3.b*/
from
t1, t2, t3
where
t1.a = t2.a (+)
and t2.b = t3.b (+)
and t3.b is null
order by t1.a
i get only one row
A A_1 B
2 2 null
My question is simple how can i only by changing columns getting back change number of returned rows? I must say i dont expect these result i expect two rows again.
Thanks for help.

Answer 1

BluShadow wrote:
I think I know what you are getting at.
By testing for null on t3.b when you aren't selecting the column, you are enforcing oracle to perform the join through t2 onto t1, but Oracle can't join because t2 has no matching row (although it's outer joined to t1) and therefore, for the one row it can't actually determine if t3.b is null or not, so that row can't match the conditions in a "true" sense and be displayed. If you select the column then oracle can test its nullness ok. (Perhaps this is a bug, I don't know, it's just how I know it works)If you get different results only by changing the projection part of the query this is a bug and nothing else. I can't reproduce using Oracle 10g XE, I get in both cases shown the expected two rows.
What versions are you using to test this?
SQL>
SQL> select * from v$version
  2  where rownum <= 1;
BANNER
Oracle Database 10g Express Edition Release 10.2.0.1.0 - Product
SQL>
SQL> drop table t1 purge;
Table dropped.
SQL> drop table t2 purge;
Table dropped.
SQL> drop table t3 purge;
Table dropped.
SQL>
SQL> create table t1 (a number not null);
Table created.
SQL> create table t2 (a number, b number);
Table created.
SQL> create table t3 (b number);
Table created.
SQL>
SQL> insert into t3 values (1);
1 row created.
SQL> insert into t3 values (2);
1 row created.
SQL> insert into t3 values (3);
1 row created.
SQL>
SQL> insert into t1 (a) values (1);
1 row created.
SQL> insert into t2 (a,b) values (1,1);
1 row created.
SQL>
SQL> insert into t1 (a) values (2);
1 row created.
SQL> insert into t2 (a,b) values (2, null);
1 row created.
SQL>
SQL> insert into t1 (a) values (3);
1 row created.
SQL>
SQL> insert into t1 (a) values (4);
1 row created.
SQL> insert into t2 (a,b) values (4,1);
1 row created.
SQL>
SQL> insert into t1 (a) values (5);
1 row created.
SQL> insert into t2 (a,b) values (5,3);
1 row created.
SQL>
SQL> commit;
Commit complete.
SQL>
SQL> select t1.a
  2       , t2.a, t2.b
  3       , t3.b
  4  from
  5         t1 left outer join t2 on (t1.a = t2.a)
  6            left outer join t3 on (t2.b = t3.b)
  7  where t3.b is null
  8  order by t1.a;
         A          A          B          B
         2          2
         3
SQL>
SQL> select t1.a
  2       , t2.a, t2.b
  3  --     , t3.b
  4  from
  5         t1 left outer join t2 on (t1.a = t2.a)
  6            left outer join t3 on (t2.b = t3.b)
  7  where t3.b is null
  8  order by t1.a;
         A          A          B
         2          2
         3
SQL>Regards,
Randolf
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