Question

given the following query

SET @num :=0, @current_shop_id := NULL, @current_product_id := NULL;

SELECT * FROM (

SELECT products.shop_id, products.product_id, @num := IF(@current_shop_id=shops.shop_id, IF(@current_product_id=products.product_id,@num,@num+1),0) AS row_number, @current_shop_id := shops.shop_id AS shop_dummy, @current_product_id := products.product_id AS product_dummy

    FROM
    favorites fav1 INNER JOIN
    products ON
    fav1.product_id=products.product_id AND 
    fav1.current=1 AND
    fav1.closeted=1 AND 
    fav1.user_id=30  INNER JOIN

    shops ON
    shops.shop_id = products.shop_id

    ORDER BY shops.shop ASC, products.product_id DESC

) AS rowed_results WHERE rowed_results.row_number>=0 AND rowed_results.row_number<(20) AND shop_id=130

I am expecting row_number values of the form

+---------+------------+------------+------------+---------------+
| shop_id | product_id | row_number | shop_dummy | product_dummy |
+---------+------------+------------+------------+---------------+
|     130 |    1153746 |          0 |        130 |       1153746 |
|     130 |    1153736 |          1 |        130 |       1153736 |
|     130 |    1139944 |          2 |        130 |       1139944 |
|     130 |    1098296 |          3 |        130 |       1098296 |
|     130 |    1017455 |          4 |        130 |       1017455 |
|     130 |     551953 |          5 |        130 |        551953 |
|     130 |     551914 |          6 |        130 |        551914 |
+---------+------------+------------+------------+---------------+

(ie, all unique product_id values associated with a given shop_id value get a unique row number, starting from 0). Instead, I am getting

+---------+------------+------------+------------+---------------+
| shop_id | product_id | row_number | shop_dummy | product_dummy |
+---------+------------+------------+------------+---------------+
|     130 |    1153746 |          1 |        130 |       1153746 |
|     130 |    1153736 |          0 |        130 |       1153736 |
|     130 |    1139944 |          0 |        130 |       1139944 |
|     130 |    1098296 |          0 |        130 |       1098296 |
|     130 |    1017455 |          0 |        130 |       1017455 |
|     130 |     551953 |          1 |        130 |        551953 |
|     130 |     551914 |          0 |        130 |        551914 |
+---------+------------+------------+------------+---------------+

what am I doing wrong?

EDIT:

Michael Berkowski’s solution (use three queries, one for retrieving data, one for adding row numbers, one for limiting by row) works (syntax slightly changed):

SET @num :=0, @current_shop_id := NULL, @current_product_id := NULL;

#limit by row

SELECT * FROM (

    #add row

    SELECT limit_query.*, @num := IF(@current_shop_id=shop_id, IF(@current_product_id=product_id,@num,@num+1),0) AS row_number, @current_shop_id := shop_id AS shop_dummy, @current_product_id := product_id AS product_dummy FROM (

        #retrieve data

        SELECT row_query.* FROM (

            SELECT products.shop_id, products.product_id

            FROM
            favorites fav1 INNER JOIN
            products ON
            fav1.product_id=products.product_id AND 
            fav1.current=1 AND
            fav1.closeted=1 AND 
            fav1.user_id=30  INNER JOIN

            shops ON
            shops.shop_id = products.shop_id

        ) AS row_query ORDER BY shop_id ASC, product_id DESC

    ) AS limit_query

) AS rowed_results WHERE rowed_results.row_number>=0 AND rowed_results.row_number<(20) AND shop_id=130;

Purely as a matter of educational interest, I’m wondering why it is not possible to add the row numbers and limit by row number at the same time, as in the following (non-functional) example

SET @num :=0, @current_shop_id := NULL, @current_product_id := NULL;

#limit by row

SELECT rowed_results.*, @num := IF(@current_shop_id=shop_id, IF(@current_product_id=product_id,@num,@num+1),0) AS row_number, @current_shop_id := shop_id AS shop_dummy, @current_product_id := product_id AS product_dummy FROM (

    #retrieve data

    SELECT row_query.* FROM (

        SELECT products.shop_id, products.product_id

        FROM
        favorites fav1 INNER JOIN
        products ON
        fav1.product_id=products.product_id AND 
        fav1.current=1 AND
        fav1.closeted=1 AND 
        fav1.user_id=30  INNER JOIN

        shops ON
        shops.shop_id = products.shop_id

    ) AS row_query ORDER BY shop_id ASC, product_id DESC

) AS rowed_results WHERE row_number>=0 AND row_number<(20) AND shop_id=130;

Answer 1

To get the rows to increment, it should be as simple a matter as moving the @num calculation into the outer query. And in that situation, the @current_product_id should not be needed as @num can increment directly.

SET @num :=0, @current_shop_id := NULL, @current_product_id := NULL;

SELECT 
  *,
  /* Perform the row increment in the outer query so it acts on the final rowset */
  @num := @num+1 AS row_number
FROM (
    SELECT products.shop_id, products.product_id, @current_shop_id := shops.shop_id AS shop_dummy, @current_product_id := products.product_id AS product_dummy

    FROM
    favorites fav1 INNER JOIN
    products ON
    fav1.product_id=products.product_id AND 
    fav1.current=1 AND
    fav1.closeted=1 AND 
    fav1.user_id=30  INNER JOIN

    shops ON
    shops.shop_id = products.shop_id

    ORDER BY shops.shop ASC, products.product_id DESC

) AS rowed_results 
WHERE
  rowed_results.row_number>=0
  AND rowed_results.row_number<(20)
  AND shop_id=130