Deleting duplicate record from table - SQL query

Question

I need to delete duplicate rows only from the table, like I have 3 duplicate rows in the table, my query will delete 2 rows from 3 duplicated rows.

How can I get this? P

Answer 1

DELETE FROM `mytbl`
    INNER JOIN (
        SELECT 1 FROM `mytbl`
        GROUP BY `duplicated_column` HAVING COUNT(*)=2
    ) USING(`id`)

Edit:

My bad, the above query won't work.

Assuming table structure:

id int auto_increment

num int # <-- this is the column with duplicated values

The following query would work in MySQL (i checked):

DELETE `mytbl` FROM `mytbl` 
    INNER JOIN 
    (
        SELECT `num` FROM `mytbl`
        GROUP BY `num` HAVING COUNT(*)=2
    ) AS `tmp` USING (`num`)

The query would delete the rows that have 2 (not more or else) duplicated values in the num column.

Edit (again):

I suggest to add a key on the num column.

Edit(#3):

In case that the author wanted to delete the duplicated rows, the following should work for MySQL (it worked for me):

DELETE `delete_duplicated_rows` FROM `delete_duplicated_rows`
    NATURAL JOIN (
        SELECT *
        FROM `delete_duplicated_rows`
        GROUP BY `num1` HAVING COUNT(*)=2
    ) AS `der`

While assuming table structure is:

CREATE TABLE `delete_duplicated_rows` (
  `num1` tinyint(4) DEFAULT NOT NULL,
  `num2` tinyint(4) DEFAULT NOT NULL
) ENGINE=MyISAM;