sql删除除2个重复项外的所有重复项

Question

I want to be able to limit the amount of duplicate records in a mySQL database table to 2.

(Excluding the id field which is auto increment)

My table is set up like

id    city      item
---------------------
1     Miami     4
2     Detroit   5
3     Miami     4
4     Miami     18
5     Miami     4

So in that table, only row 5 would be deleted.

How can I do this?

Answer 1

MySQL has some foibles when reading and writing to the same table. So I don't actually know if this will work, the syntax is fine in many implementations of SQL, but I don't know if it's MySQL friendly...

DELETE
  yourTable
WHERE
  1 < (SELECT COUNT(*)
       FROM yourTable as Lookup
       WHERE city = yourTable.city AND item = yourTable.item AND id < yourTable.id)

EDIT

Amazingly convoluted, but worth a try?

DELETE
  yourTable
FROM
  yourTable
INNER JOIN
(
  SELECT
    id
  FROM
  (
    SELECT
      id
    FROM
      yourTable
    WHERE
      1 < (SELECT COUNT(*)
           FROM yourTable as Lookup
           WHERE city = yourTable.city AND item = yourTable.item AND id < yourTable.id)
  )
    AS inner_deletes
)
  AS deletes
    ON deletes.id = yourTable.id

Answer 2

我认为您的问题在于，您的代码和/或表结构都允许插入重复项，并且您在问何时应该真正修复数据库和/或代码。

Answer 3

i think a better solution is avoid allow more than 5 registers, you have to implement a validation where if select count(*) > 3 you will not accept the new insert.

because if you want to do this into the data tier, you have to use a stored procedure , because first you need to identify all the register with more than 3 registers and delete only the last . Saludos

Answer 4

Due to MySQL being notoriously difficult when it comes to updating queried tables (see for example the answers from Dems), the best I can figure out is sadly more than one statement but on the plus side fairly readable;

CREATE TEMPORARY TABLE Dump AS SELECT id FROM table1 WHERE id NOT IN 
  (SELECT MIN(id) FROM table1 GROUP BY city,item UNION
   SELECT MAX(id) FROM table1 GROUP BY city,item);

DELETE FROM table1 where id in (select * from Dump);

DROP TABLE DUMP;

Not sure if it was important which duplicate was removed, this keeps the first and last.

Answer 5

In your reply to Joachim's answer, you ask about saving 3 or 5 rows, this is one way to accomplish it. Depending on how you are using this database, you could either call this in a loop, or you could turn it into a stored procedure. Either way, you would continue to run this entire block of code until Rows Affected = 0:

drop table if exists TempTable;
create table TempTable
select city, item,
       count(*) as record_count, 
       min(id) as ItemToDrop -- this could be changed to max() if you 
                             -- want to delete new stuff instead 
from YourTable
group by city, item
having count(*) > 2; -- This value = number of rows you save

delete from YourTable
where id in (select ItemToDrop from TempTable);