Problems with natural join

Question

Hi!
I'm learning SQL and I have lot of doubts but I think with this example I can generalize them.
I have the tables:
book {idbook (PK), namebook}
auhor {idauthor (PK), nameauthor, idcountry (FK)}
authorship {idauthor (FK), idbook (FK)} (both the same constraint PK)
country {idcountry (PK), namecountry}
I want the name of the books that have authors from Canada.
I assumed that a correct query would be:
SELECT namebook FROM book NATURAL JOIN (authorship NATURAL JOIN (author NATURAL JOIN country)) WHERE country.namecountry = 'Canada';
The result that I expected was:
Book3
Book5
but this query returns me all books that have relations in authorship (with authors from any country), 2 times!! like:
book2
book3
book4
book5
book2
book3
book4
book5
the best I can do to get my correct result is:
SELECT namebook FROM book NATURAL JOIN (authorship NATURAL JOIN author) WHERE author.idcountry = 2;
But of course I can't use this one...
Does anyone can explain me what is happening?
Thanks a lot!
Edited by: user12040235 on 15/10/2009 09:37
Edited by: user12040235 on 15/10/2009 09:51

Answer 1

Hi,
That may be a bug.
I get the correct results (2 rows) in Oracle 10.1, but I get the same bad results you do (12 rows) in Oracle 11.1.
In Oracle 11, I get the expected results if say "SELECT *" instead of "SELECT book.namebook".
I also get the correct results if I add any of the join columns to the SELECT clause. Adding a non-join column, e.g.
SELECT  BOOK.namebook, author.nameauthor
FROM            book
...gets the wrong results.
For the benefit of anyone who wants to try this:
DROP TABLE     author;
create table     author     AS
SELECT  1 AS idauthor, 'Jose Luiz do Rego' AS nameauthor, 1 AS idcountry     FROM dual     UNION ALL
SELECT         2, 'Barbara Bela',                                      2          FROM dual     UNION ALL
SELECT         3, 'Juan Domingues',                                    5          FROM dual     UNION ALL
SELECT         4, 'José Mauro de Vasconcelos',                         1          FROM dual     UNION ALL
SELECT         5, 'Vader',                                             2          FROM dual     UNION ALL
SELECT         6, 'navathe',                                           4          FROM dual     UNION ALL
SELECT         7, 'Machado de Assis',                                  1          FROM dual
drop table     AUTHORSHIP;
CREATE TABLE     authorship     AS
SELECT         2 AS idauthor,          5 AS idbook     FROM dual     UNION ALL
SELECT         1 AS idauthor,          1 AS idbook     FROM dual     UNION ALL
SELECT         5 AS idauthor,          3 AS idbook     FROM dual     UNION ALL
SELECT         6 AS idauthor,          2 AS idbook     FROM dual     UNION ALL
SELECT         7 AS idauthor,          4 AS idbook     FROM dual     UNION ALL
SELECT         7 AS idauthor,          6 AS idbook     FROM dual;
drop table     book;
CREATE TABLE   book     AS
SELECT         1 AS idbook, 'book1' AS namebook     FROM dual     UNION ALL
SELECT         2 AS idbook, 'book2' AS namebook     FROM dual     UNION ALL
SELECT         3 AS idbook, 'book3' AS namebook     FROM dual     UNION ALL
SELECT         4 AS idbook, 'book4' AS namebook     FROM dual     UNION ALL
SELECT         5 AS idbook, 'book5' AS namebook     FROM dual     UNION ALL
SELECT         6 AS idbook, 'book6' AS namebook     FROM dual     UNION ALL
SELECT         7 AS idbook, 'book7' AS namebook     FROM dual;
DROP TABLE     country;
CREATE TABLE     country     AS
SELECT         1 AS idcountry, 'Brazil' as namecountry     FROM dual     UNION ALL
SELECT         2 AS idcountry, 'Canada' as namecountry     FROM dual     UNION ALL
SELECT         3 AS idcountry, 'Chile' as namecountry     FROM dual     UNION ALL
SELECT         4 AS idcountry, 'Venezuela' as namecountry    FROM dual     UNION ALL
SELECT         5 AS idcountry, 'USA' as namecountry          FROM dual     UNION ALL
SELECT         6 AS idcountry, 'Argentina' as namecountry    FROM dual     ;