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@EvaF Thanks, but it doesn't work. The catalog page shows "No products found".

I've noticed 2 typos in your query (SELET instead of SELECT on the first line, and idproduct instead of id_product on the last one), but still doesn't work.  

Additional info, but probably irrelevant: I tried to simulate your query and run it directly in phpMyAdmin like this:

SELECT 
  * 
FROM 
  ps_product_shop 
  JOIN (
    SELECT 
      aps.id_product, 
      concat(
        aps.warehouse + 100000, asa.quantity + 100000
      ) as warehousequantity 
    FROM 
      ps_product_shop aps 
      INNER JOIN ps_stock_available asa ON aps.id_product = asa.id_product
  ) ws 
WHERE 
  ps_product_shop.id_product = ws.id_product 
ORDER BY 
  warehousequantity DESC

• I've added SELECT * FROM ps_product_shop to the beginning
• I've replaced p.id_product with ps_product_shop.id_product in WHERE clause
• I've added ORDER BY warehousequantity DESC to the end

And it produces the correct results. I must be doing something wrong in MySQL.php then.