what's fails in this query MYSQL

The following query only work when I have a reference in the table wm_purchased_products.purchased_article_id but when this is empty mysql_num_rows return 0

The query is:

  SELECT (SUM(wm_products_quantities.new_quantity) - SUM(wm_purchased_products.purchased_article_total) ) AS stock_restante, 
         wm_products_wall.nombre, 
         wm_products_wall.detalles, 
         wm_products_wall.price, 
         wm_products_wall.image_full, 
         wm_products_wall.fecha, 
         wm_products_wall.article_hashid
    FROM wm_products_wall,
         wm_prod开发者_如何学编程ucts_quantities, 
         wm_purchased_products 
   WHERE wm_products_wall.categoria = '$new_rquery_xp' 
     AND wm_products_wall.article_hashid = wm_products_quantities.hashid_ref 
     AND wm_products_wall.article_hashid = wm_purchased_products.purchased_article_id 
GROUP BY wm_products_wall.article_hashid 
ORDER BY stock_restante ASC

How to build this query to work when I have no record in the table wm_purchased_products.purchased_article_id

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Your code FROM wm_products_wall, wm_products_quantities, wm_purchased_products means all three table are INNER JOIN with each other.(that is, each and every row in the first table is joined to each and every row in the second table and so on.)

You can let wm_products_wall LEFT JOIN with wm_purchased_products, so do wm_purchased_products.

SELECT 
(SUM(wm_products_quantities.new_quantity) - SUM(wm_purchased_products.purchased_article_total) ) AS stock_restante, 
wm_products_wall.nombre, wm_products_wall.detalles, wm_products_wall.price, 
wm_products_wall.image_full, wm_products_wall.fecha, 
wm_products_wall.article_hashid
FROM wm_products_wall
LEFT OUTER JOIN wm_products_quantities
  ON wm_products_wall.article_hashid = wm_products_quantities.hashid_ref 
LEFT OUTER JOIN wm_purchased_products
  ON wm_products_wall.article_hashid = wm_purchased_products.purchased_article_id
WHERE wm_products_wall.categoria = '$new_rquery_xp' 
GROUP BY wm_products_wall.article_hashid 
ORDER BY stock_restante ASC