Delete duplicate rows (don't delete all duplicate)

Try the steps described in this article: Removing duplicates from a PostgreSQL database.

It describes a situation when you have to deal with huge amount of data which isn't possible to group by.

A simple solution would be this:

DELETE FROM foo
       WHERE id NOT IN (SELECT min(id) --or max(id)
                        FROM foo
                        GROUP BY hash)

Where hash is something that gets duplicated.

delete from table
where not id in 
(select max(id) from table group by [duplicate row])

This is random (max Value) choice which row you need to keep. If you have aggre whit this please provide more details

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