Deleting duplicate rows with sql

Question

I am trying to delete duplicate rows from my mysql table. I\'ve tried multiple queries but I am keep on getting this error: #1093 - You can\'t specify target table \'usa_cit

Answer 1

Judging from your examples, when you say "duplicate", you mean "having the same combination of id_state and city_name", correct? If so after you have done removing the duplictes, I strongly suggest creating a UNIQUE constraint on {id_state, city_name}.

To actually remove the duplicates, it is not enough to just identify the set of duplicates, you must also decide which of the identified duplicates to keep. Assuming you want to keep the ones with the smallest id, the following piece of SQL will do the job:

CREATE TEMPORARY TABLE usa_city_to_delete AS
    SELECT id FROM usa_city T1
    WHERE EXISTS (
        SELECT * FROM usa_city T2
        WHERE
            T1.id_state = T2.id_state 
            AND T1.city_name = T2.city_name
            AND T1.id > T2.id
    );

DELETE FROM usa_city
WHERE id IN (SELECT id FROM usa_city_to_delete);

DROP TEMPORARY TABLE usa_city_to_delete;

Unfortunately, MySQL does not allow the correlated subqueries in DELETE, otherwise we could have done that in a single statement, without the temporary table.

--- EDIT ---

You can't have a correlated subquery but you can have JOIN, as illustrated by Carlos Quijano answer. Also, the temporary table can be created implicitly, as suggested by Kokers.

So it is possible to do it in a single statement, contrary to what I wrote above...