Question

I am trying to select DISTINCT products from within categories with category ids (1, 5, 12), ORDERED by cat_order + prod_order from MySQL database

The problem:

if a product is found in more than 1 category I need to show the first result,

ie: product number 1 is assigned to categories 1 and 5, I need to display product number 1 from category 1 along with its prod_order and skip the listing in category 5,

• essentually I need to display all products from category 1, than move on to category 5 and display all products from there, where product id was not shown previously, and move on to another category in the list (12)

if I run something like:

SELECT
    prod_to_cat.prod_id AS prod_to_cat_prod_id,
    prod_to_cat.prod_order AS prod_to_cat_prod_order,
    prod_to_cat.cat_id AS prod_to_cat_cat_id,
    prod_to_cat.cat_order AS prod_to_cat_cat_order,
    products.id,
    products.name

FROM
    prod_to_cat, products

WHERE
    prod_to_cat.prod_id = products.id
AND prod_to_cat.cat_id IN (1, 5, 12)
GROUP BY 
    prod_to_cat.prod_id
ORDER BY
    prod_to_cat_cat_order ASC,
    prod_to_cat_prod_order DESC

I get inconsistent results (product 1 will not be selected from the first category in the list), that is why I opted to select without “GROUP BY prod_id” and wrap that with another select which than groups by prod_id.

like so:

SELECT
    prod_to_cat_prod_id,
    prod_to_cat_prod_order,
    prod_to_cat_cat_id,
    name
FROM
    (
        SELECT
            prod_to_cat.prod_id AS prod_to_cat_prod_id,
            prod_to_cat.prod_order AS prod_to_cat_prod_order,
            prod_to_cat.cat_id AS prod_to_cat_cat_id,
            prod_to_cat.cat_order AS prod_to_cat_cat_order,
            products.id,
            products.name
        FROM
            prod_to_cat, products
        WHERE
            prod_to_cat.prod_id = products.id
        AND prod_to_cat.cat_id IN (1, 5, 12)
        ORDER BY 
            prod_to_cat_cat_order ASC,
            prod_to_cat_prod_order DESC
    ) AS prod
GROUP BY
    prod_to_cat_prod_id
ORDER BY
    prod_to_cat_cat_order ASC,
    prod_to_cat_prod_order DESC
LIMIT 0, 10;

What I am trying to do:

I am trying to find a more efficiant way to do this.

Table structure:

prod_to_cat:

prod_id | cat_id | cat_order | prod_order   |
    1        1          1           2
    2        1          1           0
    3        1          1           0
    1        5          2           4
    4        5          2           0


products:


id  |   name |  descr   |   price   |
 1    name_1
 2    name_2
 3    name_3
 4    name_4

each product can be in any number of categories, for example product id 1 is in categories id 1 and 5 in the example above.

Thanks a lot for any replies.
Pasha

Answer 1

You want the groupwise minimum:

SELECT   prod_to_cat.*,
         products.name
FROM     prod_to_cat NATURAL JOIN (
           SELECT   prod_id,
                    MIN(cat_id) AS cat_id
           FROM     prod_to_cat
           WHERE    cat_id IN (1, 5, 12)
           GROUP BY prod_id
         ) t
    JOIN products ON t.prod_id = products.id
ORDER BY prod_to_cat.cat_order  ASC,
         prod_to_cat.prod_order DESC

See it on sqlfiddle.