How to delete duplicates on a MySQL table?

Question

I need to DELETE duplicated rows for specified sid on a MySQL table.

How can I do this with an SQL query?

Answer 1

Another easy way... using UPDATE IGNORE:

U have to use an index on one or more columns (type index). Create a new temporary reference column (not part of the index). In this column, you mark the uniques in by updating it with ignore clause. Step by step:

Add a temporary reference column to mark the uniques:

ALTER TABLE `yourtable` ADD `unique` VARCHAR(3) NOT NULL AFTER `lastcolname`;

=> this will add a column to your table.

Update the table, try to mark everything as unique, but ignore possible errors due to to duplicate key issue (records will be skipped):

UPDATE IGNORE `yourtable` SET `unique` = 'Yes' WHERE 1;

=> you will find your duplicate records will not be marked as unique = 'Yes', in other words only one of each set of duplicate records will be marked as unique.

Delete everything that's not unique:

DELETE * FROM `yourtable` WHERE `unique` <> 'Yes';

=> This will remove all duplicate records.

Drop the column...

ALTER TABLE `yourtable` DROP `unique`;