如何正确使用 Oracle ORDER BY 和 ROWNUM?
我很难将存储过程从 SQL Server 转换为 Oracle，以使我们的产品与之兼容.
I am having a hard time converting stored procedures from SQL Server to Oracle to have our product compatible with it.
我有一些查询会根据时间戳返回某些表的最新记录:
I have queries which returns the most recent record of some tables, based on a timestamp :
SQL Server:
SELECT TOP 1 *
FROM RACEWAY_INPUT_LABO
ORDER BY t_stamp DESC
=> 这将返回我最近的记录
=> That will returns me the most recent record
但是甲骨文:
SELECT *
FROM raceway_input_labo
WHERE rownum <= 1
ORDER BY t_stamp DESC
=> 这将返回最旧的记录(可能取决于索引)，无论 ORDER BY 语句如何！
=> That will returns me the oldest record (probably depending on the index), regardless the ORDER BY statement!
我以这种方式封装了 Oracle 查询以符合我的要求:
I encapsulated the Oracle query this way to match my requirements:
SELECT *
FROM
(SELECT *
FROM raceway_input_labo
ORDER BY t_stamp DESC)
WHERE rownum <= 1
它的工作原理.但这对我来说听起来像是一个可怕的黑客，特别是如果我在涉及的表中有很多记录.
and it works. But it sounds like a horrible hack to me, especially if I have a lot of records in the involved tables.
实现这一目标的最佳方法是什么?
What is the best way to achieve this ?
推荐答案
where 语句在 order by 之前执行.因此，您想要的查询是取第一行，然后按 t_stamp desc 排序".这不是你想要的.
The where statement gets executed before the order by. So, your desired query is saying "take the first row and then order it by t_stamp desc". And that is not what you intend.
子查询方法是在 Oracle 中执行此操作的正确方法.
The subquery method is the proper method for doing this in Oracle.
如果你想要一个在两台服务器上都能运行的版本，你可以使用:
If you want a version that works in both servers, you can use:
select ril.*
from (select ril.*, row_number() over (order by t_stamp desc) as seqnum
from raceway_input_labo ril
) ril
where seqnum = 1
外部 * 将在最后一列返回1".您需要单独列出列以避免这种情况.
The outer * will return "1" in the last column. You would need to list the columns individually to avoid this.
相关文章