SELECT 
  c.company_id 
FROM 
  cscart_products_categories pc 
  LEFT JOIN cscart_categories c ON c.category_id = pc.category_id 
WHERE 
  pc.product_id = 504 
GROUP BY 
  c.company_id 
LIMIT 
  2

Query time 0.00048

Explain
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE pc ref pt pt 3 const 17 Using temporary; Using filesort
1 SIMPLE c eq_ref PRIMARY,p_category_id,idx_category_id PRIMARY 3 mahm3t_cs443.pc.category_id 1

Result

company_id
1
2