SELECT pp.id, pp.name, pp.ref, pp.image, pp.availability,
		    -- ps.id as size_av, 
		    pp.promo, pp.price, pp.price_promo
	      FROM products_params pp
	      -- LEFT JOIN products_params_sizes ps ON pp.id = ps.param_id AND ps.quantity > 0
	      WHERE pp.product_id = 
	      GROUP BY pp.id
	      

You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'GROUP BY pp.id' at line 7