Deleting duplicate rows with sql - mysql

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_city' for update in FROM clause
The table looks like this:
usa_city
--------
id(pk)
id_state
city_name
And the queries I have tired were:
DELETE FROM usa_city
WHERE id NOT IN
(
SELECT MIN(id)
FROM usa_city
GROUP BY city_name, id_state
)
And:
DELETE
FROM usa_city
WHERE usa_city.id IN
-- List 1 - all rows that have duplicates
(SELECT F.id
FROM usa_city AS F
WHERE Exists (SELECT city_name, id_state, Count(id)
FROM usa_city
WHERE usa_city.city_name = F.city_name
AND usa_city.id_state = F.id_state
GROUP BY usa_city.city_name, usa_city.id_state
HAVING Count(usa_city.id) > 1))
AND usa_city.id NOT IN
-- List 2 - one row from each set of duplicate
(SELECT Min(id)
FROM usa_city AS F
WHERE Exists (SELECT city_name, id_state, Count(id)
FROM usa_city
WHERE usa_city.city_name = F.city_name
AND usa_city.id_state = F.id_state
GROUP BY usa_city.city_name, usa_city.id_state
HAVING Count(usa_city.id) > 1)
GROUP BY city_name, id_state);
Thanks in advance.

Answer 1

Try to select the duplicates first, the delete them
DELETE FROM usa_city WHERE city_id IN
(
SELECT city_id FROM usa_city
GROUP BY city_name, id_state
HAVING count(city_id) > 1
)
Hope it helps!!!
MODIFIED: Based on the comment, if you want to keep one record, you can make a join and keep the lowest value
DELETE c1 FROM usa_city c1, usa_city c2 WHERE c1.id < c2.id AND
(c1.city_name= c2.city_name AND c1.id_state = c2.id_state)
Be sure to make a backup before executing the query above...

Answer 2

from mysql documentation:
"Currently, you cannot delete from a table and select from the same
table in a subquery."
but here is a workaround for update, should work for delete too.
also, you could select rows, and then in php for example delete them in loop

Answer 3

You may found here an answer to your problem: How to delete duplicate records in mysql database?
You should improve your database by using keyfields to prevent duplicate rows, so you dont need to clear in future.

Answer 4

Edit : This solution is also found if you follow the link posted by BloodyWorld, so if it works please go and upvote DMin's post here
Found this browsing the internet (#1 google result for mysql delete duplicate rows), have you tried it?
delete from table1
USING table1, table1 as vtable
WHERE (NOT table1.ID=vtable.ID)
AND (table1.field_name=vtable.field_name)

Answer 5

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...