MySQL SELECT, JOIN, GROUP BY query optimizing - mysql

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.