问题
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.
回答1:
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
回答2:
Interesting!
However, I would never use a set operator (UNION, UNION ALL, INTERSECT, MINUS) together with an asterisk (*).
The order of columns can change, maybe not by you but by somebody doing maintenance on the database, or by migrating your database to a new system using export/import, etc. Simple example:
CREATE TABLE t (a INT, b INT, c INT);
SELECT * FROM t;
A B C
ALTER TABLE t MODIFY b INVISIBLE;
ALTER TABLE t MODIFY b VISIBLE;
SELECT * FROM t;
A C B
来源：https://stackoverflow.com/questions/61611705/oracle-12-2-01-selecting-columns-from-different-tables-with-similar-names-in