I'm trying to create a CTE, which I think is working fine. However, I want to then run an update query on this CTE but I keep getting the following error in MySQL WorkBench,
"Error Code: 1288. The target table pptest of the UPDATE is not updatable"
I've had a look around but no not understand any of the work arounds and on top of that I'm not too clued up on my MySQL either. My goal was basically to create a table/view with a bunch of records in there that were partitioned and would have an index number on each row, which would be represented by "row_num". This was to group duplicated data in a table together and then I was hoping to simply run an update query on this structure where the row_num was greater than 1. So simple logic but I cannot figure out any other way of achieving my goal. Can anyone help with this please?
The full query I ran before was,
WITH cte as (select *, row_number() over (partition by col_a order by col_a) row_num from db_name.table_name)
update cte set col_b='test' where row_num > 1
This syntax is not supported in MySQL. Actually this looks like SQL Server syntax... You can't just port queries from one database to another and expect them to work.
In MySQL, you could use the update ... join syntax. However, you need a primary key column (or set of columns), that uniquely identifies each row, to serve as a join condition. Assuming that column id is your primary key, that would be:
update db_name.table_name t
inner join (
select id
row_number() over (partition by col_a order by col_a) row_num
from db_name.table_name)
) t1 on t1.id = t.id
set t.col_b = 'test'
where t1.row_num > 1
Side notes:
As I understand this, your query is meant to flag duplicates on col_a. But the way you use row_number(), it is undefined which of the duplicate rows will be flagged, because your order by clause is not deterministic; you would better have an over() clause like: over (partition by col_a order by id)
window functions such as row_number() are available in MySQL 8.0 only
Update and Delete not work in mysql temporary table. A work around is to join the original table with temporary table CTE and update original table :
WITH cte as (select *, row_number()
over (partition by col_a order by col_a) row_num
from db_name.table_name)
update db_name.table_name T1
Join
cte on T1.col_a = cte.col_a set t1.col_b='test' where cte.row_num > 1
this work fine for delete instruction too.
I have a MySQL table which stores backup log entries from a large number of devices (currently about 750). I need to use a query to get details of the last entry for each device. I am currently using a nested query to achieve this, which was working fine initially. However the table now has thousands of rows and the query is taking a long time to run. I would like to improve the performance of the query, and would like to know if this is possible through the use of joins instead of a nested select statement, or through some other improvement I could make.
The current query is:
SELECT id, pcname, pcuser, checkTime as lastCheckTime,
TIMESTAMPDIFF(SECOND,checkTime,now()) as lastCheckAge,
lastBackup, diff, threshold, backupStatus,
TIMESTAMPDIFF(SECOND,lastBackup,now()) as ageNow
FROM checkresult
WHERE (checkTime, pcname) IN
(
SELECT max(checkTime), pcname
FROM checkresult
GROUP BY pcname
)
ORDER BY id desc;
id is the primary key for the table, and is the only indexed column.
The table uses InnoDB
Try using an explicit join instead:
SELECT id, checkresult.pcname, pcuser, checkTime as lastCheckTime,
TIMESTAMPDIFF(SECOND,checkTime,now()) as lastCheckAge,
lastBackup, diff, threshold, backupStatus,
TIMESTAMPDIFF(SECOND,lastBackup,now()) as ageNow
FROM checkresult join
(SELECT pcname, max(checkTime) as maxct
FROM checkresult
GROUP BY pcname
) pm
on checkresult.pcname = pm.pcname and checkresult.checkTime = pm.maxct
ORDER BY id desc;
I have a database with many entries, starting with ID 100000 and ending at ID 200000.
Now I'm trying to set the first and lowest ID to 1, and decrease on each next ID +1
So the new first ID would be 1, and the last and highest would be 99000 in that example.
Is there any mysql query, which updates the rows from a given number, and +1 for each next row?
You can use a row number query in update and join with same table but update id with row number result from joined result set
update t
join (
select id,#r:=#r+1 `row`
from t
cross join (select #r:=0) t1
) t2 on (t.id = t2.id)
set t.id = t2.row
Note: donot directly run this query on your production database first test it with a copy of your database if result is successful
then apply on production database
Fiddle Demo
im having trouble with update if selected table is true...
IF this select statement is true
(SELECT COUNT(*) FROM Add_bonus WHERE value > 0) > 0)
THEN do
UPDATE bonus SET amount = amount + 10 WHERE 1
The if statement is only allowed in stored procedures, functions, and triggers in MySQL. You can do this with a single update:
UPDATE bonus
SET amount = amount + 10
WHERE exists (select 1 from Add_Bonus where value > 0);
Note that this query will update all rows in the bonus table when any appropriate row exists in the add_bonus table. That appears to be the intention of your original query. More commonly, there might be an employee id linking the two tables.
Also note the use of exists. This is typically more efficient than using count(*), because it can stop processing on the first row that matches.
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.
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:
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.
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.
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.
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.
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 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
If your id is incremental then use something like
delete from table where id < (select max(id) from table)-N
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
DELETE FROM table WHERE ID NOT IN
(SELECT MAX(ID) ID FROM table)
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.
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 ?
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
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]
DELETE FROM table WHERE id NOT IN (
SELECT id FROM table ORDER BY id, desc LIMIT 0, 10
)
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
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
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.