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 records :

id  | name   | surname |
1   | test   | one     |
2   | test   | two     |
3   | test3  | three   |
4   | test7  | four    |
5   | test   | five    |
6   | test11 | eleven  |

Without duplicates :

id  | name   | surname |
1   | test   | one     |
3   | test3  | three   |
4   | test7  | four    |
6   | test11 | eleven  |

I’ve googled this but it seems not to be working :

DELETE  ct1
FROM    mytable ct1
        , mytable ct2
WHERE   ct1.name = ct2.name 
        AND ct1.id < ct2.id 

ERROR:  syntax error at or near "ct1"
LINE 1: DELETE  ct1
                ^

********** Error **********

I’m using postgres database.

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…