How to SELECT the newest four items per category?

Question

I have a database of items. Each item is categorized with a category ID from a category table. I am trying to create a page that lists every category, and underneath each

Answer 1

In other databases you can do this using the ROW_NUMBER function.

SELECT
    category_id, image, date_listed
FROM
(
    SELECT
        category_id, image, date_listed,
        ROW_NUMBER() OVER (PARTITION BY category_id
                           ORDER BY date_listed DESC) AS rn
    FROM item
) AS T1
WHERE rn <= 4

Unfortunately MySQL does not support the ROW_NUMBER function, but you can emulate it using variables:

SELECT
    category_id, image, date_listed
FROM
(
    SELECT
        category_id, image, date_listed,
        @rn := IF(@prev = category_id, @rn + 1, 1) AS rn,
        @prev := category_id
    FROM item
    JOIN (SELECT @prev := NULL, @rn = 0) AS vars
    ORDER BY category_id, date_listed DESC
) AS T1
WHERE rn <= 4

See it working online: sqlfiddle

It works as follows:

• Intially @prev is set to NULL, and @rn is set to 0.
• For each row we see, check if the category_id is the same as the previous row.
  • If yes, increment the row number.
  • Otherwise start a new category and reset the row number back to 1.
• When the subquery completes, the final step is to filter so that only rows with row number less than or equal to 4 are kept.