How to select maximum 3 items per users in MySQL?

Question

I run a website where users can post items (e.g. pictures). The items are stored in a MySQL database.

I want to query for the last ten posted items BUT with the con

Answer 1

I would first select 10 distinct users, then selecting images from each of those users with a LIMIT 3, possibly by a union of all those and limit that to 10.

That would atleast narrow down the data you need to process to a fair amount.

Answer 2

It's pretty easy with a correlated sub-query:

SELECT `img`.`id` , `img`.`userid`
FROM `img`
WHERE 3 > (
SELECT count( * )
FROM `img` AS `img1`
WHERE `img`.`userid` = `img1`.`userid`
AND `img`.`id` > `img1`.`id` )
ORDER BY `img`.`id` DESC
LIMIT 10 

The query assumes that larger id means added later

Correlated sub-queries are a powerful tool! :-)

Answer 3

This is difficult because MySQL does not support the LIMIT clause on sub-queries. If it did, this would be rather trivial... But alas, here is my naïve approach:

SELECT
  i.UserId,
  i.ImageId
FROM
  UserSuppliedImages i
WHERE
  /* second valid ImageId */
  ImageId = (
    SELECT MAX(ImageId)
    FROM UserSuppliedImages
    WHERE UserId = i.UserId
  )
  OR
  /* second valid ImageId */
  ImageId = (
    SELECT MAX(ImageId)
    FROM   UserSuppliedImages
    WHERE UserId = i.UserId
      AND ImageId < (
        SELECT MAX(ImageId)
        FROM UserSuppliedImages
        WHERE UserId = i.UserId
      )
    )
  /* you get the picture... 
     the more "per user" images you want, the more complex this will get */
LIMIT 10;

You did not comment on having a preferred result order, so this selects the latest images (assuming ImageId is an ascending auto-incrementing value).

For comparison, on SQL Server the same would look like this:

SELECT TOP 10
  img.ImageId,
  img.ImagePath,
  img.UserId
FROM
  UserSuppliedImages img
WHERE
  ImageId IN (
    SELECT TOP 3 ImageId
    FROM UserSuppliedImages 
    WHERE UserId = img.UserId
  )