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.
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]
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
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 Nth 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.
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.
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
)
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!
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
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
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
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 ?
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.
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.
DELETE FROM table WHERE id NOT IN (
SELECT id FROM table ORDER BY id, desc LIMIT 0, 10
)
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.
DELETE FROM table WHERE ID NOT IN
(SELECT MAX(ID) ID FROM table)
If your id is incremental then use something like
delete from table where id < (select max(id) from table)-N
Source: Stackoverflow.com