SQL 查询：删除表中除最新N条记录外的所有记录？

Question

Is it possible to build a single mysql query (without variables) to remove all records from the table, except latest N (sorted by id desc)?

Something like this, only it doesn't work:)

delete from table order by id ASC limit ((select count(*) from table ) - N)

Thanks.

Answer 1

You cannot delete the records that way, the main issue being that you cannot use a subquery to specify the value of a LIMIT clause.

This works (tested in MySQL 5.0.67):

DELETE FROM `table`
WHERE id NOT IN (
  SELECT id
  FROM (
    SELECT id
    FROM `table`
    ORDER BY id DESC
    LIMIT 42 -- keep this many records
  ) foo
);

The intermediate subquery is required. Without it we'd run into two errors:

1. SQL Error (1093): You can't specify target table 'table' for update in FROM clause - MySQL doesn't allow you to refer to the table you are deleting from within a direct subquery.
2. SQL Error (1235): This version of MySQL doesn't yet support 'LIMIT & IN/ALL/ANY/SOME subquery' - You can't use the LIMIT clause within a direct subquery of a NOT IN operator.

Fortunately, using an intermediate subquery allows us to bypass both of these limitations.

---

Nicole has pointed out this query can be optimised significantly for certain use cases (such as this one). I recommend reading that answer as well to see if it fits yours.

Answer 2

I know I'm resurrecting quite an old question, but I recently ran into this issue, but needed something that scales to large numbers well . There wasn't any existing performance data, and since this question has had quite a bit of attention, I thought I'd post what I found.

The solutions that actually worked were the Alex Barrett's double sub-query/ NOT IN method (similar to Bill Karwin's ), and Quassnoi's LEFT JOIN method.

Unfortunately both of the above methods create very large intermediate temporary tables and performance degrades quickly as the number of records not being deleted gets large.

What I settled on utilizes Alex Barrett's double sub-query (thanks!) but uses <= instead of NOT IN :

DELETE FROM `test_sandbox`
  WHERE id <= (
    SELECT id
    FROM (
      SELECT id
      FROM `test_sandbox`
      ORDER BY id DESC
      LIMIT 1 OFFSET 42 -- keep this many records
    ) foo
  )

It uses OFFSET to get the id of the th record and deletes that record and all previous records.

Since ordering is already an assumption of this problem ( ORDER BY id DESC ), <= is a perfect fit.

It is much faster, since the temporary table generated by the subquery contains just one record instead of N records.

Test case

I tested the three working methods and the new method above in two test cases.

Both test cases use 10000 existing rows, while the first test keeps 9000 (deletes the oldest 1000) and the second test keeps 50 (deletes the oldest 9950).

+-----------+------------------------+----------------------+
|           | 10000 TOTAL, KEEP 9000 | 10000 TOTAL, KEEP 50 |
+-----------+------------------------+----------------------+
| NOT IN    |         3.2542 seconds |       0.1629 seconds |
| NOT IN v2 |         4.5863 seconds |       0.1650 seconds |
| <=,OFFSET |         0.0204 seconds |       0.1076 seconds |
+-----------+------------------------+----------------------+

What's interesting is that the <= method sees better performance across the board, but actually gets better the more you keep, instead of worse.

Answer 3

Unfortunately for all the answers given by other folks, you can't DELETE and SELECT from a given table in the same query.

DELETE FROM mytable WHERE id NOT IN (SELECT MAX(id) FROM mytable);

ERROR 1093 (HY000): You can't specify target table 'mytable' for update 
in FROM clause

Nor can MySQL support LIMIT in a subquery. These are limitations of MySQL.

DELETE FROM mytable WHERE id NOT IN 
  (SELECT id FROM mytable ORDER BY id DESC LIMIT 1);

ERROR 1235 (42000): This version of MySQL doesn't yet support 
'LIMIT & IN/ALL/ANY/SOME subquery'

The best answer I can come up with is to do this in two stages:

SELECT id FROM mytable ORDER BY id DESC LIMIT n; 

Collect the id's and make them into a comma-separated string:

DELETE FROM mytable WHERE id NOT IN ( ...comma-separated string... );

(Normally interpolating a comma-separate list into an SQL statement introduces some risk of SQL injection, but in this case the values are not coming from an untrusted source, they are known to be integer values from the database itself.)

note: Though this doesn't get the job done in a single query, sometimes a more simple, get-it-done solution is the most effective.

Answer 4

DELETE  i1.*
FROM    items i1
LEFT JOIN
        (
        SELECT  id
        FROM    items ii
        ORDER BY
                id DESC
        LIMIT 20
        ) i2
ON      i1.id = i2.id
WHERE   i2.id IS NULL

Answer 5

如果你的 id 是增量的，那么使用类似的东西

delete from table where id < (select max(id) from table)-N

Answer 6

To delete all the records except te last N you may use the query reported below.

It's a single query but with many statements so it's actually not a single query the way it was intended in the original question.

Also you need a variable and a built-in (in the query) prepared statement due to a bug in MySQL.

Hope it may be useful anyway...

nnn are the rows to keep and theTable is the table you're working on.

I'm assuming you have an autoincrementing record named id

SELECT @ROWS_TO_DELETE := COUNT(*) - nnn FROM `theTable`;
SELECT @ROWS_TO_DELETE := IF(@ROWS_TO_DELETE<0,0,@ROWS_TO_DELETE);
PREPARE STMT FROM "DELETE FROM `theTable` ORDER BY `id` ASC LIMIT ?";
EXECUTE STMT USING @ROWS_TO_DELETE;

The good thing about this approach is performance : I've tested the query on a local DB with about 13,000 record, keeping the last 1,000. It runs in 0.08 seconds.

The script from the accepted answer...

DELETE FROM `table`
WHERE id NOT IN (
  SELECT id
  FROM (
    SELECT id
    FROM `table`
    ORDER BY id DESC
    LIMIT 42 -- keep this many records
  ) foo
);

Takes 0.55 seconds. About 7 times more.

Test environment: mySQL 5.5.25 on a late 2011 i7 MacBookPro with SSD

Answer 7

DELETE FROM table WHERE ID NOT IN
(SELECT MAX(ID) ID FROM table)

Answer 8

try below query:

DELETE FROM tablename WHERE id < (SELECT * FROM (SELECT (MAX(id)-10) FROM tablename ) AS a)

the inner sub query will return the top 10 value and the outer query will delete all the records except the top 10.

Answer 9

Just wanted to throw this into the mix for anyone using Microsoft SQL Server instead of MySQL. The keyword 'Limit' isn't supported by MSSQL, so you'll need to use an alternative. This code worked in SQL 2008, and is based on this SO post. https://stackoverflow.com/a/1104447/993856

-- Keep the last 10 most recent passwords for this user.
DECLARE @UserID int; SET @UserID = 1004
DECLARE @ThresholdID int -- Position of 10th password.
SELECT  @ThresholdID = UserPasswordHistoryID FROM
        (
            SELECT ROW_NUMBER()
            OVER (ORDER BY UserPasswordHistoryID DESC) AS RowNum, UserPasswordHistoryID
            FROM UserPasswordHistory
            WHERE UserID = @UserID
        ) sub
WHERE   (RowNum = 10) -- Keep this many records.

DELETE  UserPasswordHistory
WHERE   (UserID = @UserID)
        AND (UserPasswordHistoryID < @ThresholdID)

Admittedly, this is not elegant. If you're able to optimize this for Microsoft SQL, please share your solution. Thanks!

Answer 10

If you need to delete the records based on some other column as well, then here is a solution:

DELETE
FROM articles
WHERE id IN
    (SELECT id
     FROM
       (SELECT id
        FROM articles
        WHERE user_id = :userId
        ORDER BY created_at DESC LIMIT 500, 10000000) abc)
  AND user_id = :userId

Answer 11

DELETE FROM table WHERE id NOT IN (
    SELECT id FROM table ORDER BY id, desc LIMIT 0, 10
)

Answer 12

This should work as well:

DELETE FROM [table] 
INNER JOIN (
    SELECT [id] 
    FROM (
        SELECT [id] 
        FROM [table] 
        ORDER BY [id] DESC
        LIMIT N
    ) AS Temp
) AS Temp2 ON [table].[id] = [Temp2].[id]

Answer 13

What about :

SELECT * FROM table del 
         LEFT JOIN table keep
         ON del.id < keep.id
         GROUP BY del.* HAVING count(*) > N;

It returns rows with more than N rows before. Could be useful ?

Answer 14

Stumbled across this and thought I'd update. This is a modification of something that was posted before . I would have commented, but unfortunately don't have 50 reputation...

LOCK Tables TestTable WRITE, TestTable as TestTableRead READ;
DELETE FROM TestTable
WHERE ID <= (
  SELECT ID
  FROM TestTable as TestTableRead -- (the 'as' declaration is required for some reason)
  ORDER BY ID DESC LIMIT 1 OFFSET 42 -- keep this many records);
UNLOCK TABLES;

The use of 'Where' and 'Offset' circumvents the sub-query. You also cannot read and write from the same table in the same query, as you may modify entries as they're being used. The Locks allow to circumvent this. This is also safe for parallel access to the database by other processes. For performance and further explanation see the linked answer.

Tested with mysql Ver 15.1 Distrib 10.5.18-MariaDB

For further details on locks, see here

Answer 15

Using id for this task is not an option in many cases. For example - table with twitter statuses. Here is a variant with specified timestamp field.

delete from table 
where access_time >= 
(
    select access_time from  
    (
        select access_time from table 
            order by access_time limit 150000,1
    ) foo    
)

Answer 16

Answering this after a long time...Came across the same situation and instead of using the answers mentioned, I came with below -

DELETE FROM table_name order by ID limit 10

This will delete the 1st 10 records and keep the latest records.

Answer 17

Why not

DELETE FROM table ORDER BY id DESC LIMIT 1, 123456789

Just delete all but the first row (order is DESC!), using a very very large nummber as second LIMIT-argument.See here