ORDER BY and LIMIT in GROUP BY - mysql

Question

I'm trying to get a subset of records in a GROUP BY, I've seen a lot of crazy solutions out there, but they just seem too complicated, is there any more efficient way to do this.
SELECT user_id, GROUP_CONCAT(item_id ORDER BY `timestamp`) AS items
FROM wb_user_book_current_item GROUP BY user_id
So this will return me all the current items for all users which is okay so far. But I only want the ten most recent items. Adding ORDER BY to the GROUP_CONCAT helps, but it still doesn't give me the last ten records.
EDIT
If I do something like this and hard code the user_id then I can get the results I want for that one user, problem is combining it so that I don't need to hard code the user_id and can for instance just get ALL users last ten items
SELECT GROUP_CONCAT(cp2.item_id) AS items
FROM (SELECT cp.user_id, cp.item_id
FROM wb_user_book_current_item cp
WHERE cp.user_id=1 ORDER BY cp.`timestamp`
LIMIT 10) AS cp2
GROUP BY cp2.user_id

Answer 1

This is a difficult problem, but how about this:
SELECT user_id, GROUP_CONCAT(item_id ORDER BY `timestamp`) AS items
FROM wb_user_book_current_item T
WHERE NOT EXISTS
(
SELECT 1
FROM wb_user_book_current_item T2
WHERE T2.user_id = T.user_id
ORDER BY T2.`timestamp` DESC
LIMIT 10,1
)
OR T.`timestamp` > (
SELECT T2.`timestamp`
FROM wb_user_book_current_item T2
WHERE T2.user_id = T.user_id
ORDER BY T2.`timestamp` DESC
LIMIT 10,1
)
GROUP BY user_id
This of course assumes you won't have two rows with the same timestamp for the same user.
If your timestamp field is always a positive integer, you can also replace the NOT EXISTS...OR with a COALESCE:
SELECT user_id, GROUP_CONCAT(item_id ORDER BY `timestamp`) AS items
FROM wb_user_book_current_item T
WHERE T.`timestamp` > COALESCE((
SELECT T2.`timestamp`
FROM wb_user_book_current_item T2
WHERE T2.user_id = T.user_id
ORDER BY T2.`timestamp` DESC
LIMIT 10,1
), 0)
GROUP BY user_id
Original answer, but apparently MySQL doesn't understand how to do this properly and complains the subselect returns multiple rows. Of course we want multiple rows; it's a GROUP_CONCAT. Grr.
Unfortunately, I think there's no real way around using a subquery:
SELECT T.user_id,
GROUP_CONCAT((SELECT T2.item_id
FROM wb_user_book_current_item T2
WHERE T2.user_id = T.user_id
ORDER BY T2.`timestamp`
LIMIT 10)) AS items
FROM wb_user_book_current_item T
GROUP BY user_id
Otherwise, adding LIMIT anywhere else will either limit the number of groups, or limit from the total recordset over the table (and not the group) - neither of which are what you are trying to achieve.

Answer 2

So came across a nice solution here that works pretty well.
http://www.xaprb.com/blog/2006/12/07/how-to-select-the-firstleastmax-row-per-group-in-sql/
It's something like this put all together:
SET #num := 0, #user_id := '';
SELECT cp2.user_id, CONCAT(cp2.item_id) AS items
FROM (
SELECT cp.user_id, cp.item_id,
#num := IF(#user_id = cp.user_id, #num + 1, 1) AS row_number,
#user_id := cp.user_id AS dummy
FROM wb_user_curent_item AS cp
ORDER BY cp.user_id ASC, cp.`timestamp` DESC
) AS cp2 WHERE cp2.row_number <= 10
GROUP BY cp2.user_id
So basically it just uses the num increment to limit the records rather than using LIMIT

Answer 3

SELECT
i.user_id,
GROUP_CONCAT(i.item_id ORDER BY i.timestamp) AS items
FROM
( SELECT DISTINCT user_id
FROM wb_user_book_current_item
) AS du
JOIN
wb_user_book_current_item AS i
ON i.user_id = du.user_id
AND i.timestamp <= COALESCE(
( SELECT i2.item_id
FROM wb_user_book_current_item AS i2
WHERE i2.user_id = du.user_id
ORDER BY i2.timestamp ASC
LIMIT 1 OFFSET 9
)
, '2038-01-19 03:14:07')
GROUP BY
i.user_id ;
An index on (user_id, timestamp, item_id) will help efficiency.

Answer 4

Try this:
SELECT
user_id,
GROUP_CONCAT(item_id ORDER BY `timestamp`) AS items
FROM wb_user_book_current_item
GROUP BY user_id
LIMIT 0, 10

Answer 5

UPDATE: I didn't notice the GROUP_CONCAT so you will have to use sub query in conunction with LIMIT
use LIMIT
SELECT column_name(s)
FROM table_name
LIMIT number