How to find duplicated field values and delete them in MySQL

Question

Consider we have a field named (username) in our table named (tpl_users), now this table has lots of duplicated rows

I wrote this code to delete duplicated usernames:

Delete FROM tpl_users WHERE username = username;

How is it possible to delete duplicated usernames?

Answer 1

Your query deletes all the rows where the user name is not NULL.

If you want to identify the user names which are associated with more than one row:

SELECT username
  FROM tpl_users
 GROUP BY username
HAVING COUNT(*) > 1;

Before converting that into a DELETE, you need to be aware that the majority of queries will delete all the records associated with a duplicated name, rather than retaining just one of the duplicates.

Answer 2

What you want to do is delete duplicate rows.

You'll do this by finding all non-duplicate rows, and replacing the table with those rows:

create table tpl_users_new as select distinct * from tpl_users;

alter table tpl_users rename to tpl_users_old;

alter table tpl_users_new rename to tpl_users;

Answer 3

Assuming you have a primary key column in the table, you can do this:

DELETE
FROM tpl_uers
USING tpl_users, tpl_users AS vtable
WHERE vtable.id > tpl_users.id
AND tpl_users.username = vtable.username

Answer 4

If you want to keep the users with the lowest id, first make sure this query has what you want to remove (and backup your database):

SELECT u1.id, u1.username FROM tpl_users u1 LEFT JOIN tpl_users u2 ON u1.username = u2.username WHERE u1.id > u2.id;

Then, if your database is backed up, and you're sure the above statement represents what you want to remove.

DELETE u1 FROM tpl_users u1 LEFT JOIN tpl_users u2 ON u1.username = u2.username WHERE u1.id > u2.id