Inner join is not removing duplicates!!

Question

Table name is customers
     CNUM     CNAME     CITY     RATING     SNUM
1     2001     Hoffman     London     100     1001
2     2002     Giovanni     Rome     200     1003
3     2003     Liu     San Jose     200     1002
4     2004     Grass     Berlin     300     1002
5     2006     Clemens     London     100     1001
6     2008     Cisneros     San Jose     300     1007
7     2007     Perira     Rome     100     1004
Query is
To Find all pairs of customers haveing the same rating
I have tried:
SELECT c.cname,cb.cname FROM customers c INNER JOIN customers cb ON c.rating=cb.rating AND c.cname!=cb.cname
     CNAME     CNAME
1     Perira     Hoffman
2     Clemens     Hoffman
3     Liu     Giovanni
4     Giovanni     Liu
5     Cisneros     Grass
6     Perira     Clemens
7     Hoffman     Clemens
8     Grass     Cisneros
9     Clemens     Perira
10     Hoffman     Perira
Ideally it should till 5 rows but it should not return such rows for an e.g. Hoffman Perira since I got Perira Hoffman already. Your repsonse will be appreciated.
Thanks,
Vaibhav

Answer 1

Hi, Vaibhav,
914683 wrote:
Table name is customers
     CNUM     CNAME     CITY     RATING     SNUM
1     2001     Hoffman     London     100     1001
2     2002     Giovanni     Rome     200     1003
3     2003     Liu     San Jose     200     1002
4     2004     Grass     Berlin     300     1002
5     2006     Clemens     London     100     1001
6     2008     Cisneros     San Jose     300     1007
7     2007     Perira     Rome     100     1004
Query is
To Find all pairs of customers haveing the same rating
I have tried:
SELECT c.cname,cb.cname FROM customers c INNER JOIN customers cb ON c.rating=cb.rating AND c.cname!=cb.cnameMaybe what you want is
SELECT      c.cname
,         cb.cname
FROM          customers  c
INNER JOIN  customers  cb  ON   c.rating  = cb.rating
                                   AND  c.cname       < cb.cname     -- not !=
     CNAME     CNAME
1     Perira     Hoffman
2     Clemens     Hoffman
3     Liu     Giovanni
4     Giovanni     Liu
5     Cisneros     Grass
6     Perira     Clemens
7     Hoffman     Clemens
8     Grass     Cisneros
9     Clemens     Perira
10     Hoffman     PeriraThe title you chose for this thread is "inner join is not removing duplicates!!".
Inner joins (or any other kinds of joins) are not supposed to remove duplicates. Use SELECT DISTINCT to remove duplicates, that is, completely identical rows.
You don't have any duplicates. For example, these rows from your result set:
`        CNAME     CNAME
1     Perira     Hoffman
10     Hoffman     Periraare not identical; in fact, both columns are different.
Ideally it should till 5 rows but it should not return such rows for an e.g. Hoffman Perira since I got Perira Hoffman already. Please post the exact output you want from that sample data. (As Hoek said, you should post CREATE TABLE and INSERT statements for the sample data, too.)