Question

I have duplicate IP Address records in my database like this :

id | ipaddress
1    192.168.xxx.xxx
2    192.168.xxx.xxx
3    111.118.xxx.xxx
4    111.118.xxx.xxx

I want unique ip addresses in my field. How should I delete all duplicate entry?

Thanks

Answer 1

Deleting duplicates in MySQL is a bit tricky because of the stupid restriction that the table cannot be referenced in a sub-select. Therefor the sub-select needs to be re-written into a join:

DELETE d
FROM mytable d
LEFT JOIN (
   SELECT min(id) as min_id
   FROM   mytable
   GROUP BY trim(ipaddress)
) tokeep ON tokeep.min_id = d.id
WHERE keep.min_id IS NULL;

SQLFiddle demo: http://sqlfiddle.com/#!2/9cfb9c/1

Edit

There is actually a way to get around the stupid sub-select restriction. If the table is wrapped into a derived table inside the sub-select, the MySQL parser doesn’t notice this and happily deletes with a sub-select:

delete mt 
from mytable mt
where exists (
    select * 
    from (
      select id, ipaddress
      from mytable
    ) ex
    where TRIM(ex.ipaddress) = TRIM(mt.ipaddress)
   and ex.id < mt.id
)