Delete all but one duplicate record

Question

I have a table that is supposed to keep a trace of visitors to a given profile (user id to user id pair). It turns out my SQL query was a bit off and is producing multiple pairs instead of single ones as intended. With hindsight I should have enforced a unique constraint on each id+id pair.

Now, how could I go about cleaning up the table? What I want to do is delete all duplicate pairs and leave just one.

So for example change this:

23515 -> 52525 date_visited
23515 -> 52525 date_visited
23515 -> 52525 date_visited
12345 -> 54321 date_visited
12345 -> 54321 date_visited
12345 -> 54321 date_visited
12345 -> 54321 date_visited
23515 -> 52525 date_visited
...

Into this:

23515 -> 52525 date_visited
12345 -> 54321 date_visited

Update: Here is the table structure as requested:

id  int(10)         UNSIGNED    Non     Aucun   AUTO_INCREMENT
profile_id  int(10)         UNSIGNED    Non     0 
visitor_id  int(10)         UNSIGNED    Non     0
date_visited    timestamp           Non     CURRENT_TIMESTAMP   

Answer 1

Use group by in a subquery:

delete from my_tab where id not in 
(select min(id) from my_tab group by profile_id, visitor_id);

You need some kind of unique identifier(here, I'm using id).

UPDATE

As pointed out by @JamesPoulson, this causes a syntax error in MySQL; the correct solution is (as shown in James' answer):

delete from `my_tab` where id not in
( SELECT * FROM 
    (select min(id) from `my_tab` group by profile_id, visitor_id) AS temp_tab
);

Answer 2

Select all unique rows
Copy them to a new temp table
Truncate original table
Copy temp table data to original table

That's what I'd do. I'm not sure if there's 1 query that would do all this for you.

Answer 3

This will work:

With NewCTE
AS
(
Select *, Row_number() over(partition by ID order by ID)as RowNumber from 
table_name
)
Delete from NewCTE where RowNumber > 1

Answer 4

Here's Frank Schmitt's solution with a small workaround for the temporary table:

delete from `my_tab` where id not in
( SELECT * FROM 
    (select min(id) from `my_tab` group by profile_id, visitor_id) AS temp_tab
)