Delete all but one duplicate record

问题

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

回答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
);

回答2:

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
)

回答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

回答4:

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.

回答5:

If you are using SQL you can manually delete the duplicate rows keeping one entry just follows the procedure 1. Go into your table where you have duplicate data. 2. Apply the filter to segregate duplicate data for each individual id 3. Select all the rows you want to delete. 4. Press delete and save the result. 5. Repeat the process for each id you have duplicate entries for. it's a long procedure but you can see the results immediately in real-time.

Hope this solution worked for you!!

来源：https://stackoverflow.com/questions/5882649/delete-all-but-one-duplicate-record

标签

sql

sql-delete