Question

I’m looking to find a way to search a table for duplicate values and return those duplicates (or even just one of the set of duplicates) as the result set.

For instance, let’s say I have these data:

uid    |   semi-unique id
1      |       12345
2      |       21345
3      |       54321
4      |       41235
5      |       12345
6      |       21345

I need to return either:

12345
12345
21345
21345

Or:

12345
21345

I’ve tried googling around and keep coming up short. Any help please?

Answer 1

To get each row, you can use window functions:

select t.*
from (select t.*, count(*) over (partition by [semi-unique id]) as totcnt
      from t
     ) t
where totcnt > 1

To get just one instance, try this:

select t.*
from (select t.*, count(*) over (partition by [semi-unique id]) as totcnt,
             row_number() over (partition by [semi-unique id] order by (select NULL)
                               ) as seqnum
      from t
     ) t
where totcnt > 1 and seqnum = 1

The advantage of this approach is that you get all the columns, instead of just the id (if that helps).