MySQL: finding duplicates across multiple fields

Question

Let's say I had a MySQL database with the following five records in a table:

ID: 1
Field1: A
Field2: B
Field3: C

ID: 2
Field1: D
Field2: E
Field3: F

ID: 3
Field1: A
Field2: H
Field3: I

ID: 4
Field1: J
Field2: K
Field3: A

ID: 5
Field1: M
Field2: D
Field3: O

Notice that the following values are duplicated:

ID 1, field 1 has the same value as ID 3, field 1 and ID 4, field 3.

ID 2, field 1 has the same value as ID 5, field 2.

Is there a SELECT statement that could find all of the above duplicates?

Kalendae · Accepted Answer · 2011-01-08 05:30:50Z

it is doable but not sure if it is any more efficient than just doing it at the application level:

your table:

mysql> select * from test;
+----+--------+--------+--------+
| id | field1 | field2 | field3 |
+----+--------+--------+--------+
|  1 | A      | B      | C      |
|  2 | D      | E      | F      |
|  3 | A      | H      | I      |
|  4 | J      | K      | A      |
|  5 | M      | D      | O      |
+----+--------+--------+--------+
5 rows in set (0.00 sec)

the select to find dupes:

mysql> select count(value) as dupe_count,value from (select field1 as value from test union all select field2 from test union all select field3 from test) as tbl group by value having count(value) > 1 order by 1 desc;
+------------+-------+
| dupe_count | value |
+------------+-------+
|          3 | A     |
|          2 | D     |
+------------+-------+
2 rows in set (0.00 sec)

basically you union the three columns into one then look for dupes

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