MySQL Adding COUNT( ) values - mysql

Question

EDIT: SQL Fiddle here
I'm working on a product feed. I get a list of offers;
Every offer has 1 product
Every product belongs to 1 category
A category can be a subcategory to another category
I have three tables (of which i will only show you the relevant rows)
Offers:
OFFERS
___________________
| id | product_id |
-------------------
| 1 | 16 |
-------------------
| 2 | 54 |
-------------------
| 3 | 52 |
-------------------
| 4 | 20 |
-------------------
| 5 | 7 |
-------------------
| 6 | 5 |
-------------------
Products:
PRODUCTS
_______________
| id | cat_id |
---------------
| 16 | 1 |
---------------
| 54 | 3 |
---------------
| 52 | 4 |
---------------
| 20 | 1 |
---------------
| 7 | 15 |
---------------
| 5 | 3 |
---------------
Categories:
CATEGORIES
_____________________________________________________________
| id | display_name | original_name | subcat_of | is_active |
-------------------------------------------------------------
| 1 | Cars | automobiles | 0 | 1 |
-------------------------------------------------------------
| 2 | | motorcycles | 0 | 0 |
-------------------------------------------------------------
| 3 | Muscle cars | muscle-cars | 1 | 1 |
-------------------------------------------------------------
| 4 | Hybrid cars | treehugwagons | 1 | 1 |
-------------------------------------------------------------
I have to write two queries. The first one needs to
return the names and count the amount of offers for a given main category and its subcategories
but only if that main category is active
and if a category has no display_name, use the original_name
I think i have this one down:
SELECT
offers.id AS offer_id,
product_id,
products.cat_id,
CASE
WHEN categories.display_name <> ''
THEN categories.display_name
ELSE categories.original_name
END AS cat_name,
COUNT(offers.id) as num_offers
FROM
offers
INNER JOIN
products
ON
product_id = products.id
INNER JOIN
categories
ON
cat_id = categories.id
WHERE
categories.is_active = 1
AND
(categories.id = :cat_id OR categories.subcat_of = :cat_id)
GROUP BY
cat_name
ORDER BY
cat_name ASC
I'm pretty sure this query is far from ideal, but for now, it works.
It is the second query I need that gives me problems. That one needs to:
return the names and count the amount of offers for a given main category and its subcategories and return the sum of those counts per main category
but only if that main category is active
and if a category has no display_name, use the original_name
I could use some PHP to do the summing myself, but I'd be surprised if something that easy could not be done in SQL.

Answer 1

I believe it is possible and fairly simple, assuming you don't have sub-sub-categories:
SELECT CASE
WHEN c_main.display_name <> ''
THEN c_main.display_name
ELSE c_main.original_name
END cat_name,
COUNT(o.id) as num_offers
FROM offers o
JOIN products p
ON o.product_id = p.id
JOIN categories c
ON p.cat_id = c.id
AND (c.id = :cat_id OR c.subcat_of = :cat_id)
/* AND c.is_active = 1 /* Include if necessary */
JOIN categories c_main
ON c_main.id = :cat_id
AND c_main.is_active = 1
GROUP BY cat_name
ORDER BY cat_name ASC
Your first query I would write as:
SELECT CASE
WHEN c.display_name <> ''
THEN c.display_name
ELSE c.original_name
END cat_name,
COUNT(o.id) as num_offers
FROM offers o
JOIN products p
ON o.product_id = p.id
JOIN categories c
ON p.cat_id = c.id
AND (c.id = :cat_id OR c.subcat_of = :cat_id)
AND c.is_active = 1
GROUP BY cat_name
ORDER BY cat_name ASC
As an aside:
I would also consider NULLing out display_names that are empty, then you can replace
CASE
WHEN c_main.display_name <> ''
THEN c_main.display_name
ELSE c_main.original_name
END cat_name
With:
COALESCE(c_main.display_name, c_main.original_name) cat_name

Answer 2

It can't be done in MySQL since MySQL doesn't support recursive queries. You have three choices:
Emulate recursive/hierarchical queries with some complex function. (Related)
Switch to SQL Server which supports recursive queries using CTE.
Write a bit of PHP.
It's up to you :)
As a side note i leave here an interesting slide on Models for Hierarchical data with SQL and PHP written by SO user #Bill Karwin which may suit your situation.