Oracle SQL non-unique table alias in one Select

Question

Does somebody know why this works with both table alias "x"?

Select x.company_name  
      ,x.employee_name 
FROM company  x
JOIN employee x ON x.company_id = 5

I know that the JOIN with id 5 makes no sense...

Thanks for the lesson!

Answer 1

I'd lay money on "Oracle bug" - Of all the things that've most held me back from adopting the ANSI JOIN syntax, it's been Oracle's painfully egregious and bug-laden implementation thereof. That said, "Failure to catch a syntax error" is pretty minor, and Oracle has, as far as I can tell, largely cleaned up their act.

Answer 2

The first two queries below are equivalent. In the ON clause of the join the table alias x only refers to the last table to use that alias so only the employee table is restricted.

In the SELECT and WHERE expressions the x alias refers to both tables - so, where the column names are unique then they can be successfully referenced but where there are identical column names then oracle raises an ORA-00918: column ambiguously defined exception (as happens in query 3 if the comment is removed).

I can't find any documentation on this but it looks very like a bug.

SQL Fiddle

Oracle 11g R2 Schema Setup:

CREATE TABLE employee (
  company_id    NUMBER(3),
  employee_name VARCHAR2(20)
);

CREATE TABLE company (
  company_id    NUMBER(3),
  company_name VARCHAR2(20)
);

INSERT INTO employee VALUES ( 4, 'Four Emp' );
INSERT INTO employee VALUES ( 5, 'Five Emp' );
INSERT INTO employee VALUES ( 6, 'Six Emp' );

INSERT INTO company VALUES ( 4, 'Four Company' );
INSERT INTO company VALUES ( 5, 'Five Company' );
INSERT INTO company VALUES ( 6, 'Six Company' );

Query 1:

SELECT *
FROM   company  x
       JOIN
       employee x
       ON x.company_id = 5

Results:

| COMPANY_ID | COMPANY_NAME | EMPLOYEE_NAME |
|------------|--------------|---------------|
|          5 | Four Company |      Five Emp |
|          5 | Five Company |      Five Emp |
|          5 |  Six Company |      Five Emp |

Query 2:

SELECT *
FROM   company x
       CROSS JOIN
       (SELECT * FROM employee WHERE company_id = 5) x

Results:

| COMPANY_ID | COMPANY_NAME | EMPLOYEE_NAME |
|------------|--------------|---------------|
|          5 | Four Company |      Five Emp |
|          5 | Five Company |      Five Emp |
|          5 |  Six Company |      Five Emp |

Query 3:

SELECT --x.company_id,
       x.company_name,
       x.employee_name
FROM   company x
       CROSS JOIN
       (SELECT * FROM employee WHERE company_id = 5) x

Results:

| COMPANY_NAME | EMPLOYEE_NAME |
|--------------|---------------|
| Four Company |      Five Emp |
| Five Company |      Five Emp |
|  Six Company |      Five Emp |