Question

Assuming that I have the below customer_offer table.

My question is:

How to select all the rows where the key(s) are duplicated in that table?

+---------+-------------+------------+----------+--------+---------------------+
| link_id | customer_id | partner_id | offer_id | key    | date_updated        |
+---------+-------------+------------+----------+--------+---------------------+
|       1 |          99 |         11 |       14 | mmmmmq | 2011-09-21 12:40:46 |
|       2 |         100 |         11 |       14 | qmmmmq | 2011-09-21 12:40:46 |
|       3 |         101 |         11 |       14 | 8mmmmq | 2011-09-21 12:40:46 |
|       4 |          99 |         11 |       14 | Dmmmmq | 2011-09-21 12:59:28 |
|       5 |         100 |         11 |       14 | Nmmmmq | 2011-09-21 12:59:28 |
+---------+-------------+------------+----------+--------+---------------------+

UPDATE:

Thanks so much for all your answer. There are many answers are good. Now I got the solution to do.

Answer 1

select * 
from customer_offer 
where key in 
(select key from customer_offer group by key having count(*) > 1)

Update:
As mentioned from @Scorpi0, if with a big table, it is better to use join. And from mysql6.0 the new optimizer will convert this kind of subqueries into joins.