A small correction in the SQL script ("{uc_product_stock}" instead "uc_product_stock" In the last entry)
SELECT pr.nid,active,n.status as status,n.sticky as sticky,pr.model as sku,n.title as title,n.type as type,stock,threshold FROM {uc_products} as pr LEFT JOIN {uc_product_stock} as s ON pr.nid = s.nid LEFT JOIN {node} as n ON pr.nid = n.nid WHERE EXISTS (SELECT * FROM {uc_product_stock} as ups where n.nid = ups.nid)