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        如何正确使用 Oracle ORDER BY 和 ROWNUM?

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                1. 本文介绍了如何正确使用 Oracle ORDER BY 和 ROWNUM?的处理方法,对大家解决问题具有一定的参考价值,需要的朋友们下面随着跟版网的小编来一起学习吧!


                  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.


                  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


                  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 * 
                      (SELECT *
                       FROM raceway_input_labo 
                       ORDER BY t_stamp DESC)
                  WHERE  rownum <= 1


                  and it works. But it sounds like a horrible hack to me, especially if I have a lot of records in the involved tables.


                  What is the best way to achieve this ?


                  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.


                  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.

                  这篇关于如何正确使用 Oracle ORDER BY 和 ROWNUM?的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持跟版网!

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