Delete duplicate rows from table

Question

I have unique keys id keys in my table but I have a column with duplicate values? how do I get rid of those, while preserving only one of them like this :

Duplicate

Answer 1

You can try this running multiple times:

delete from mytable where id in (
    select max(id)
      from mytable
     group by name
    having count(1) > 1
);

Where multiple times equals the maximum number of repetitions you have in name column.

Otherwise, you can try this more complex query:

delete from mytable where id in (
    select id from mytable
    except 
    (
    select min(id)
      from mytable
     group by name
    having count(1) > 1
    union all
    select min(id)
      from mytable
     group by name
    having count(1) = 1
    )
);

Running this query one time only should delete all you need. Haven't tried it though...

Answer 2

Using Rank, actually I'm not totally sure about the syntax because I'm not that good at PostgreSQL, this is just a hint anyway (anybody's correction will be appreciated):

DELETE FROM mytable
WHERE id NOT IN
(
   SELECT x.id FROM
   (
      SELECT id, RANK() OVER (PARTITION BY name ORDER BY id ASC) AS r
      FROM mytable
   ) x
   WHERE x.r = 1
)