ORACLE 12.2.01 selecting columns from different tables with similar names --> internal column identifier used

Question

I wrote a SELECT performing a UNION and in each UNION part using some JOINs. The tables, which are joined have partly the same column identifiers. And if a "SELECT *" is performed, ORACLE decides to display the internal column names instead of the "real" column names.

To show the effect I created two tables (with partly similar column identifiers, "TID" and "TNAME") and filled them with some data:

create table table_one (tid number(10), tname varchar2(10), t2id number(10));
create table table_two (tid number(10), tname varchar2(10));

insert into table_two values (1,'one');
insert into table_two values (2,'two');
insert into table_two values (3,'three');

insert into table_one values (1,'eins',1);
insert into table_one values (2,'zwei',2);
insert into table_one values (3,'drei',3);

The I SELECTED the columns afterwards with the following statement:

select * 
from table_one 
inner join table_two on table_two.tid = table_one.t2id
where table_one.tid = 1
union
select * 
from table_one 
inner join table_two on table_two.tid = table_one.t2id
where table_one.tid = 2;

And got this confusing result:

QCSJ_C000000000300000 QCSJ_C000000000300002       T2ID QCSJ_C000000000300001 QCSJ_C000000000300004
                    1 eins                           1                     1 one
                    2 zwei                           2                     2 two

When the statement is written with tablenames to specify the columns, everything works as I expected:

select table_one.* , table_two.*
from table_one 
inner join table_two on table_two.tid = table_one.t2id
where table_one.tid = 1
minus
select * 
from table_one 
inner join table_two on table_two.tid = table_one.t2id
where table_one.tid = 2;

   TID TNAME            T2ID        TID TNAME
     1 eins                1          1 one
     2 zwei                2          2 two

Can anybody explain that?

I expanded my tests with two more tables to prevent double usage of table in the statement:

create table table_3 (tid number(10), tname varchar2(10), t4id number(10));
create table table_4 (tid number(10), tname varchar2(10));

insert into table_4 values (1,'one');
insert into table_4 values (2,'two');
insert into table_4 values (3,'three');

insert into table_3 values (1,'eins',1);
insert into table_3 values (2,'zwei',2);
insert into table_3 values (3,'drei',3);

select * 
from table_one 
inner join table_two on table_two.tid = table_one.t2id
where table_one.tid = 1
union
select * 
from table_3
inner join table_4 on table_4.tid = table_3.t4id
where table_3.tid = 2;

select * 
from table_one 
inner join table_two on table_two.tid = table_one.t2id
where table_one.tid = 1
union
select * 
from table_3
inner join table_4 on table_4.tid = table_3.t4id
where table_3.tid = 2;

The result is the same. Oracle uses internal identifiers.

BTW, in real life, I would never use SELECT * in a UNION. Much too dangerous, the query will change or break if a new column is added to or removed from one of the tables. — wolφi
Oracle is internally using QCSJ_C000 as column alias to avoid the duplicate names and make it unique as you are using * instead of explicit column names. — Lalit Kumar B
@LalitKumarB yes, I know. But in the second statement there are dublicate column names as well. And ORACLE provides the columns with similar names ( or in some applications as Quest TOAD with _1 as TID_1 and TNAME_1). In my opinion the behaviour with the internal names as QCSJ_C000 is new..... — virtualbee
No, it's not new. DocId 2658003.1 refers to Oracle version 11.2.0.4. — wolφi
@virtualbee No, it's not a new behaviour. See the DocId "wolφi" gave. It's a pretty old and known behaviour. — Lalit Kumar B

wolφi wolφi · Accepted Answer · 2020-05-05T11:26:18

According to Oracle (DocId 2658003.1), this happens when three conditions are met:

ANSI join
UNION / UNION ALL
the same table appears more than once in the query

Aparently, "QCSJ_C" is used internally when Oracle transforms ANSI style joins.

EDIT:

Found a minimal example:

SELECT * FROM dual d1 JOIN dual d2 ON d1.dummy=d2.dummy
UNION
SELECT * FROM dual d1 JOIN dual d2 ON d1.dummy=d2.dummy;

QCSJ_C000000000300000 QCSJ_C000000000300001
X                     X

It can be fixed by either using non-ANSI join syntax:

SELECT * FROM dual d1, dual d2 WHERE d1.dummy=d2.dummy
UNION
SELECT * FROM dual d1, dual d2 WHERE d1.dummy=d2.dummy;

DUMMY DUMMY_1
X     X

Or, preferably by using column names instead of *:

SELECT d1.dummy, d2.dummy FROM dual d1 JOIN dual d2 ON d1.dummy=d2.dummy
UNION
SELECT d1.dummy, d2.dummy FROM dual d1 JOIN dual d2 ON d1.dummy=d2.dummy;

DUMMY DUMMY_1
X     X

ORACLE 12.2.01 selecting columns from different tables with similar names --> internal column identifier used

2 Answers