Question

我很困惑为什么我们使用这个min函数。我无法理解下面的代码片段如何工作。请指导

COALESCE( min((start_Date)) OVER (partition by Seq_id  ORDER BY start_Date rows between 1 following and 1 following),cast( '9999-12-31 00:00:00' as timestamp(6))  end_Date FROM table.test1

Answer 1

这是您的查询：

SELECT COALESCE(min((start_Date)) OVER (partition by Seq_id 
                                        ORDER BY start_Date
                                        rows between 1 following and 1 following
                                      ),
         cast( '9999-12-31 00:00:00' as timestamp(6))
        ) as end_Date
FROM table.test1

此查询正在执行;

SELECT COALESCE(LEAD(Start_Date) OVER (PARTITION BY seq_id ORDER BY start_date),
                cast( '9999-12-31 00:00:00' as timestamp(6))
               ) as end_Date

也就是说，它是从Start_Date定义的“下一行”中获取日期值。

我认为使用此构造是因为（某些版本）Teradata不支持LEAD()。

Answer 2

你可以在Teradata窗口函数，Preceding和Preceding之间的行之间找到很好的解释： http://pauldhip.blogspot.dk/2015/04/window-function-rows-between-preceding.html

teradata查询中前后的区别是什么

2 个答案: