Delete duplicate entries while keeping one - mysql

Question

I have a table but it has no unique ID or primary key.
It has 3 columns in total.
name
user_id
role_id
ben
1
2
ben
1
2
sam
1
3
I'd like to remove one entry with the name Ben.
So output would look like this
name
user_id
role_id
ben
1
2
sam
1
3
Most of the examples shows deleting duplicate entries with ID or primary key. However how would I retain one entry whilest removing the other ones?
Using the following query I was able to get duplicated rows
SELECT name, user_id, role_id, count(*) FROM some_table
GROUP BY name, user_id, role_id
HAVING count(*) > 1
To clarify, I am looking to delete these rows.
Prefer not creating a new table.

Answer 1

If you don't have to worry about other users accessing the table -
CREATE TABLE `new_table` AS
SELECT DISTINCT `name`, `user_id`, `role_id`
FROM `old_table`;
RENAME TABLE
`old_table` TO `backup`,
`new_table` TO `old_table`;
Or you could use your duplicates query to output lots of single row delete queries -
SELECT
`name`,
`user_id`,
`role_id`,
COUNT(*),
CONCAT('DELETE FROM some_table WHERE name=\'', `name`, '\' AND user_id=\'', `user_id`, '\' AND role_id=\'', `role_id`, '\' LIMIT 1;') AS `delete_stmt`
FROM `some_table`
GROUP BY `name`, `user_id`, `role_id`
HAVING COUNT(*) > 1;
Or you could temporarily add a SERIAL column and then remove it after the delete -
ALTER TABLE `some_table` ADD COLUMN `temp_id` SERIAL;
DELETE `t1`.*
FROM `some_table` `t1`
LEFT JOIN (
SELECT MIN(`temp_id`) `min_temp_id`
FROM `some_table`
GROUP BY `name`, `user_id`, `role_id`
) `t2` ON `t1`.`temp_id` = `t2`.`min_temp_id`
WHERE `t2`.`min_temp_id` IS NULL;
ALTER TABLE `some_table` DROP COLUMN `temp_id`;

Answer 2

Note that you are not saving anything by not having a primary key; mysql (at least with innodb) requires a primary key and will create a hidden one if you do not have one. So I would first add a primary key:
alter table some_table add id serial primary key;
Then you can easily remove duplicates with:
delete a from some_table a join some_table b on a.name=b.name and a.user_id=b.user_id and a.role_id=b.role_id and b.id < a.id;

Answer 3

I would take the duplicate records and put them into another table.
SELECT
name,
user_id,
role_id
INTO some_new_table
FROM some_table
GROUP BY name, user_id, role_id
HAVING count(*) > 1
Then you can delete those records from your source table
DELETE a
FROM some_table a
INNER JOIN some_new_table b
ON a.name = b.name
AND a.user_id = b.user_id
AND a.role_id = b.role_id
Finally you can then insert the deduped records back into your table.
INSERT INTO some_table
SELECT
name,
user_id,
role_id
FROM some_new_table
If the volume of dupes is very large you could also just create a new table with the deduped data. Truncate \ Drop the old table and then Insert \ Rename from the new table.