Need to Join 3 Tables On Different Columns

Question

product_to_category p2c contains 2 columns:

category_id product_id

This table may contain more than one entry for each product so MAX (category_id) is needed.

category_to_google c2g contains 2 columns:

category_id google_id

google_category gc contains 2 columns:

google_id name

So get MAX(category_id) FROM p2c,

get google_id FROM c2g WHERE category_id = selected category_id,

finally get name from gc WHERE google_id = selected google_id

I can't seem to get the join right.

ANSWER:

$query = $this->db->query("
    SELECT name FROM {$this->prefix}google_category 
    WHERE google_id = (
        SELECT google_id FROM {$this->prefix}category_to_google 
        WHERE category_id = (
            SELECT MAX(category_id) FROM {$this->prefix}product_to_category 
            WHERE product_id = '" . (int)$product_id . "'
        )
    )");

This works, thanks RC.

Is it possible to get this into an nice join, it has to be nested into an existing query?

Answer 1

I'm not sure I did get what you want but I think:

SELECT name FROM gc WHERE google_id = (
    SELECT google_id FROM c2g WHERE category_id = (
        SELECT MAX(category_id) FROM p2c
    )
);

might be what you are searching for.

Answer 2

Maybe something like this...

 SELECT name 
   FROM google_category c
   JOIN category_to_google cg
     ON cg.google_id = c.google_id
   JOIN (SELECT MAX(category_id) max_category_id 
           FROM product_to_category 
          WHERE product_id = $product_id) x
             ON x.max_category_id = cg.category_id