Question

I would like to try using Outer Join to find out duplicates in a table:

If a table has Primary Index ID, then the following outer join can find out the duplicate names:

mysql> select * from gifts;
+--------+------------+-----------------+---------------------+
| giftID | name       | filename        | effectiveTime       |
+--------+------------+-----------------+---------------------+
|      2 | teddy bear | bear.jpg        | 2010-04-24 04:36:03 |
|      3 | coffee     | coffee123.jpg   | 2010-04-24 05:10:43 |
|      6 | beer       | beer_glass.png  | 2010-04-24 05:18:12 |
|     10 | heart      | heart_shape.jpg | 2010-04-24 05:11:29 |
|     11 | ice tea    | icetea.jpg      | 2010-04-24 05:19:53 |
|     12 | cash       | cash.png        | 2010-04-24 05:27:44 |
|     13 | chocolate  | choco.jpg       | 2010-04-25 04:04:31 |
|     14 | coffee     | latte.jpg       | 2010-04-27 05:49:52 |
|     15 | coffee     | espresso.jpg    | 2010-04-27 06:03:03 |
+--------+------------+-----------------+---------------------+
9 rows in set (0.00 sec)

mysql> select * from gifts g1 LEFT JOIN (select * from gifts group by name) g2 
         on g1.giftID = g2.giftID where g2.giftID IS NULL;
+--------+--------+--------------+---------------------+--------+------+----------+---------------+
| giftID | name   | filename     | effectiveTime       | giftID | name | filename | effectiveTime |
+--------+--------+--------------+---------------------+--------+------+----------+---------------+
|     14 | coffee | latte.jpg    | 2010-04-27 05:49:52 |   NULL | NULL | NULL     | NULL          |
|     15 | coffee | espresso.jpg | 2010-04-27 06:03:03 |   NULL | NULL | NULL     | NULL          |
+--------+--------+--------------+---------------------+--------+------+----------+---------------+
2 rows in set (0.00 sec)

But what if the table doesn’t have a Primary Index ID, then can an outer join still be used to find out duplicates?

P.S. thanks for any non-outer-join solutions as well. If possible, I’d like to check whether it can be done using an outer join when there is no Primary ID index. thanks for helping.

Answer 1

Use EXISTS clause:

SELECT  *
FROM    gifts go
WHERE   EXISTS
        (
        SELECT  NULL
        FROM    gifts gi
        WHERE   gi.name = go.name
        LIMIT 1, 1
        )