SELECT V.* 
                    FROM t_products AS P
                    LEFT JOIN 
                        t_products_manufac AS V ON V.id = P.vyrobce
                    WHERE
                        P.kategorie IN (,4,8,1,5,2,6)
                        AND
                        P.publikovano = 1
                    GROUP BY
                        V.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 '4,8,1,5,2,6)
                        AND
                        P.publikovano' at line 6