Finding and deleting duplicate rows from a table

This tip will show you how to find duplicate rows in a table and then delete them. First, create a table called empl with columns desc and emplid. Now find out the duplicate rows in this table, using the following query:

select desc, emplid from empl 
group by desc, emplid having count(*) > 1;

To delete duplicate rows from this table, use the following query:

delete from empl a where rowid > (
select min(rowid) from empl b where
a.desc = b.desc and
a.emplid = b.emplid);

Reader Feedback

Michael P. writes: The query in this tip works very well as given. But there are times when you also want a count of the duplicates. The original query can be modified slightly to use a subquery, and thus return the count as well.

select count, desc, emplid from (
   select count(*) as count, desc, emplid
       from empl
       group by desc, emplid
  )
  where count > 1
  order by count, emplid;

This is as fast as the original and in a table with lots of duplicates (perhaps from a legacy application or a design change), the count is also nice to know.

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This was first published in August 2002

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