相关疑难解决方法(0)

如果项目已排序,我可以运行select语句并获取行号吗？

我有这样一张桌子:

mysql> describe orders;
+-------------+---------------------+------+-----+---------+----------------+
| Field       | Type                | Null | Key | Default | Extra          |
+-------------+---------------------+------+-----+---------+----------------+
| orderID     | bigint(20) unsigned | NO   | PRI | NULL    | auto_increment |
| itemID      | bigint(20) unsigned | NO   |     | NULL    |                |
+-------------+---------------------+------+-----+---------+----------------+

然后我可以运行此查询以按ID获取订单数量:

SELECT itemID, COUNT(*) as ordercount
FROM orders
GROUP BY itemID ORDER BY ordercount DESC;

这给了我itemID表中每个的计数如下:

+--------+------------+
| itemID | ordercount |
+--------+------------+
|    388 |          3 |
|    234 | …

所以我有一张表如下:

ID_STUDENT | ID_CLASS | GRADE
-----------------------------
   1       |    1     |  90
   1       |    2     |  80
   2       |    1     |  99
   3       |    1     |  80
   4       |    1     |  70
   5       |    2     |  78
   6       |    2     |  90
   6       |    3     |  50
   7       |    3     |  90

我需要对它们进行分组,排序和排序以给出:

ID_STUDENT | ID_CLASS | GRADE | RANK
------------------------------------
    2      |    1     |  99   |  1
    1      |    1     |  90   |  2
    3      |    1     |  80 …

这个查询有什么问题？它在rowno列中显示为null.

SELECT @rowno:=@rowno+1 `rn`,`id`, `title`, `topic` 
FROM stories,(SELECT @rownum:=0) r 
WHERE newstype='2';

我在'MySQL Query browser'中运行它

提前致谢.