Question

I have a temp table created from a copy from a CSV file and the result includes some duplicate ids. I need to delete any duplication. I have tried the following:

delete from my_table where id in
    (select id from (select count(*) as count, id
        from my_table group by id) as counts where count>1);

However this deletes both the duplicate records and I must keep one.

How can I delete only the 2nd record with a duplicated Id?

Thanks.

Answer 1

Your query deletes all IDs that have a count greater than 1, so it removes everything that is duplicated. What you need to do is isolate one record from the list of duplicates and preserve that:

delete
from   my_table
where  id in     (select   id
                  from     my_table
                  where    some_field in (select   some_field
                                          from     my_table
                                          group by some_field
                                          having   count(id) > 1))
and    id not in (select   min(id)
                  from     my_table
                  where    some_field in (select   some_field
                                          from     my_table
                                          group by some_field
                                          having   count(id) > 1)
                  group by some_field);

EDIT Fixed 😛