Mysql按分组获取前几条内容

$sql_set = "SET @num := 0,@GROUP := '';";
$sql = "SELECT
                *
            FROM
                (
                    SELECT
                        *, @num :=
                    IF (
                        @GROUP = `group_name`,
                        @num + 1,
                        1
                    ) AS row_number,
                    @GROUP := `group_name`
                FROM
                    v2_order
                WHERE
             group_name IS NOT NULL
                ORDER BY
                    group_name,
                    o_id DESC
                ) AS tmp
            WHERE
                tmp.row_number <= 20 ";