Question

I just found out that a table on my production server (which holds approx. 35K records) contains 588 duplicate entries in an INT(11) column which has AUTO_INCREMENT.
The UNIQUE key is missing on that column so that’s probably the cause.

Any ideas on how to give all duplicate entries a unique ID and then adding the UNIQUE key to the column so this will never happen again?

Table schema:

CREATE TABLE `items` (
 `item_ID` int(11) unsigned NOT NULL auto_increment,
 `u_ID` int(10) NOT NULL default '0',
 `user_ID` int(11) NOT NULL default '0',
 `p_ID` tinyint(4) NOT NULL default '0',
 `url` varchar(255) NOT NULL,
 `used` int(10) unsigned NOT NULL,
 `sort` tinyint(4) NOT NULL,
 `last_checked` int(11) NOT NULL,
 `unixtime` int(11) NOT NULL,
 `switched` int(11) NOT NULL,
 `active` tinyint(1) NOT NULL default '0',
 UNIQUE KEY `unique` (`p_ID`,`url`),
 KEY `index` (`u_ID`,`item_ID`,`sort`,`active`),
 KEY `index2` (`u_ID`,`switched`,`active`),
 KEY `item_ID` (`item_ID`),
 KEY `p_ID` (`p_ID`),
 KEY `u_ID` (`u_ID`)
) ENGINE=MyISAM AUTO_INCREMENT=42755 DEFAULT CHARSET=utf8

Answer 1

How about something like this? Again test it on a backup first.

# Copy duplicate records
CREATE TABLE newitem SELECT * FROM items WHERE item_ID IN 
    (SELECT item_ID FROM itemd GROUP BY item_ID HAVING COUNT(*) > 1);

# remove auto increment from id in new table
ALTER TABLE newitem DROP INDEX Item_ID, MODIFY item_ID int;

# delete duplicates from original
DELETE FROM item WHERE item_ID IN (SELECT DISTINCT item_ID FROM newitem);

#Update column to be primary key
ALTER TABLE items DROP INDEX Item_ID, ADD PRIMARY KEY (Item_ID);

# Set new duplicate ID's to null
UPDATE newitem SET item_ID=NULL;

# Insert records back into old table
INSERT INTO item SELECT * FROM newitem;

# Get rid of work table
DROP newitem;