cleaning up db of redundant data

Question

locid   country city
39409   US  Aaronsburg
128426  US  Aaronsburg
340356  US  Aaronsburg
429373  US  Aaronsburg
422717  US  Abbeville
431344  US  Abbeville
433062           


        

Answer 1

Add unique index on table location so that no duplicate records will get inserted

ALTER IGNORE TABLE location ADD UNIQUE KEY ix1(country, city);

This will automatically remove duplicate records from the table and for future insert queries you need to use INSERT IGNORE clause to avoid getting duplicate errors.

but as suggested by @AD7six in comments, it might not work on MySQL versions 5.1.41,5.5.1-m2, 6.0: see bug here

or alternate safe way to remove duplicates using DELETE query:

DELETE a
FROM location a
     LEFT JOIN (
                SELECT locid
                FROM location
                GROUP BY country, city
               )b
               ON a.locid = b.locid
WHERE b.locid IS NULL;

to resettle values of auto_increment column locid, you can just drop the primary key on locid and recreate it:

ALTER TABLE location DROP column locid;
ALTER TABLE location 
      ADD COLUMN locid INT unsigned NOT NULL AUTO_INCREMENT PRIMARY KEY FIRST;

or alternative way to resettle values of locid using UPDATE query:

SET var_locid = 0;

UPDATE location
SET locid = (@var_locid := @var_locid + 1)
ORDER BY locid ASC;