Simple query, i have a table with clientID and cardKey. One client can have many cards. Query is to find all cards belonging to client 1
I am using mysql work bench, it executes the query normally with no errors but returns no results
SELECT cID, cardKey
FROM client_cards
where `cID` = 1 ;
Your query is correct, but in the client_cards table there is no row that has cID = 1.
I want to update one column based on count of another column in same table. I tried below query but no luck.
update
table
set conntype='Multiple'
where (select COUNT(cpemac) as nos from table group by cpemac) > 1
I get following error:
1093 - You can't specify target table 'table' for update in FROM clause
In MySQL, you need to use a JOIN:
update t join
(select cpemac, count(cpemac) as nos
from t
group by cpemac
) tt
on t.cpemac = tt.cpemac
set t.conntype = 'Multiple'
where cnt > 1;
This is a specific limitation of MySQL.
I should note, however, that your version would not work in any database. It would either update all rows or no rows, depending on the result of the subquery. There is no connection between the subquery and the outer query.
I want to remove duplicates based on the combination of listings.product_id and listings.channel_listing_id
This simple query returns 400.000 rows (the id's of the rows I want to keep):
SELECT id
FROM `listings`
WHERE is_verified = 0
GROUP BY product_id, channel_listing_id
While this variation returns 1.600.000 rows, which are all records on the table, not only is_verified = 0:
SELECT *
FROM (
SELECT id
FROM `listings`
WHERE is_verified = 0
GROUP BY product_id, channel_listing_id
) AS keepem
I'd expect them to return the same amount of rows.
What's the reason for this? How can I avoid it (in order to use the subselect in the where condition of the DELETE statement)?
EDIT: I found that doing a SELECT DISTINCT in the outer SELECT "fixes" it (it returns 400.000 records as it should). I'm still not sure if I should trust this subquery, for there is no DISTINCT in the DELETE statement.
EDIT 2: Seems to be just a bug in the way phpMyAdmin reports the total count of the rows.
Your query as it stands is ambiguous. Suppose you have two listings with the same product_id and channel_id. Then what id is supposed to be returned? The first, the second? Or both, ignoring the GROUP request?
What if there is more than one id with different product and channel ids?
Try removing the ambiguity by selecting MAX(id) AS id and adding DISTINCT.
Are there any foreign keys to worry about? If not, you could pour the original table into a copy, empty the original and copy back in it the non-duplicates only. Messier, but you only do SELECTs or DELETEs guaranteed to succeed, and you also get to keep a backup.
Assign aliases in order to avoid field reference ambiguity:
SELECT
keepem.*
FROM
(
SELECT
innerStat.id
FROM
`listings` AS innerStat
WHERE
innerStat.is_verified = 0
GROUP BY
innerStat.product_id,
innerStat.channel_listing_id
) AS keepem
I have a table structure and data below.
I need to remove duplicate record from the table list. My confusion is that when I am firing query
SELECT * FROM `table` GROUP BY CONCAT(`name`,department)
then giving me correct list(12 records).
Same query when I am using the subquery:
SELECT *
FROM `table` WHERE id IN (SELECT id FROM `table` GROUP BY CONCAT(`name`,department))
It returning all record which is wrong.
So, My question is why group by in subquery is not woking.
Actually as Tim mentioned in his answer that it to get first unique record by group by clause is not a standard feature of sql but mysql allows it till mysql5.6.16 version but from 5.6.21 it has been changed.
Just change mysql version in your sql fiddle and check that you will get what you want.
In the query
SELECT * FROM `table` GROUP BY CONCAT(`name`,department)
You are selecting the id column, which is a non-aggregate column. Many RDBMS would give you an error, but MySQL allows this for performance reasons. This means MySQL has to choose which record to retain in the result set. Based on the result set in your original problem, it appears that MySQL is retaining the id of the first duplicate record, in cases where a group has more than one member.
In the query
SELECT *
FROM `table`
WHERE id IN
(
SELECT id FROM `table` GROUP BY CONCAT(`name`,department)
)
you are also selecting a non-aggregate column in the subquery. It appears that MySQL actually decides which id value to be retained in the subquery based on the id value in the outer query. That is, for each id value in table, MySQL performs the subquery and then selectively chooses to retain a record in the group if two id values match.
You should avoid using a non-aggregate column in a query with GROUP BY, because it is a violation of the ANSI standard, and as you have seen here it can result in unexpected results. If you give us more information about what result set you want, we can give you a correct query which will avoid this problem.
I welcome anyone who has documentation to support these observations to either edit my question or post a new one.
You can JOIN the grouped ids with that of table ids, so that you can get desired results.
Example:
SELECT t.* FROM so_q32175332 t
JOIN ( SELECT id FROM so_q32175332
GROUP BY CONCAT( name, department ) ) f
ON t.id = f.id
ORDER BY CONCAT( name, department );
Here order by was added just to compare directly the * results on group.
Demo on SQL Fiddle: http://sqlfiddle.com/#!9/d715a/1
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.