mysql update count query with multiple join subquery

Question

Morning, I am trying to create a count of products per category / store using an UPDATE, JOIN, SUBQUERY but I cant get the syntax right, can someone help me where I am going wrong:

UPDATE 
    oc_category_to_store c2s 
SET 
    c2s.product_count = 
        (
        SELECT 
            count(p.product_id) as product_count,
            p2c.category_id,
            p2s.store_id
        FROM 
            oc_product p
            INNER JOIN oc_product_to_category p2c ON 
                (p.product_id = p2c.product_id)
            INNER JOIN oc_product_to_store p2s ON
                (p.product_id = p2s.product_id)
        ) AS pc
WHERE
        (p2c.category_id = pc.category_id) AND
        (p2s.store_id = pc.store_id)

Essentially this is in OpenCart but the existing product count routines are done in PHP are on large DBs of products it slows the site down to a crawl.

products are linked to both categories and stores by ID using the product_to_category and product_to_store tables, categories and also linked to stores using a category_to_store table.

I want to run a mySQL query to update the product counts per category / store by storing the value in the category_to_store table as this only needs to run when products are added / deleted / updated and not every time any page loads.

CREATE TABLE `oc_product_to_category` (
  `product_id` int(11) NOT NULL,
  `category_id` int(11) NOT NULL,
  PRIMARY KEY (`product_id`,`category_id`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8 COLLATE=utf8_bin AVG_ROW_LENGTH=9 ROW_FORMAT=FIXED;

CREATE TABLE `oc_product_to_store` (
  `product_id` int(11) NOT NULL,
  `store_id` int(11) NOT NULL DEFAULT '0',
  PRIMARY KEY (`product_id`,`store_id`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8 COLLATE=utf8_bin AVG_ROW_LENGTH=9 ROW_FORMAT=FIXED;

CREATE TABLE `oc_category_to_store` (
  `category_id` int(11) NOT NULL,
  `store_id` int(11) NOT NULL,
  `product_count` int(11) DEFAULT NULL,
  PRIMARY KEY (`category_id`,`store_id`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8 COLLATE=utf8_bin AVG_ROW_LENGTH=9 ROW_FORMAT=FIXED;

Answer 1

You need to use the INNER JOIN syntax:

UPDATE 
    oc_category_to_store a
INNER JOIN 
(
    SELECT
        b.category_id,
        c.store_id,
        COUNT(*) AS productcnt
    FROM 
        oc_product a
    INNER JOIN
        oc_product_to_category b ON a.product_id = b.product_id
    INNER JOIN
        oc_product_to_store c ON a.product_id = c.product_id
    GROUP BY
        b.category_id,
        c.store_id
) b ON 
    a.category_id = b.category_id AND 
    a.store_id = b.store_id
SET
    a.product_count = b.productcnt

Answer 2

The 1st error is in your query logic. Your subquery returns 3 columns:

product_count,
p2c.category_id,
p2s.store_id

And you're trying to assign this triple to a single column *c2s.product_count*

UPDATE

The 2nd error is in the WHERE clause of your main query. You use tables p2c , p2s and pc however UPDATE clause utilizes c2s only. The most likely the WHERE clause must be in the subquery like below:

UPDATE 
    oc_category_to_store c2s 
SET 
    c2s.product_count = 
        (
        SELECT 
            count(oc_product.product_id) as product_count,
        FROM 
            oc_product p
            INNER JOIN oc_product_to_category p2c ON 
                (p.product_id = p2c.product_id)
            INNER JOIN oc_product_to_store p2s ON
                (p.product_id = p2s.product_id)
        WHERE
            (p2c.category_id = pc.category_id) AND
            (p2s.store_id = pc.store_id)        
        )