在 SQL 中查找 MAX(DATE) 值

Question

我是SQL的新手，所以这个问题-

我的 table 中有一个 DATE 列，我的查询应该使用最新的 DATE 获取值。我已经意识到我可以通过以下任何一种方式实现它-

SELECT TRUNC(MAX(createddatetm)) cdt FROM (SELECT TO_TIMESTAMP(TO_CHAR(createddate, 'Month dd, yyyy hh:mi AM'), 'Month dd, yyyy hh:mi AM') as createddatetm FROM comments WHERE commentid = '1234');
SELECT trunc(MAX(TO_TIMESTAMP(to_char(createddate, 'Month dd, yyyy hh:mi AM'), 'Month dd, yyyy hh:mi AM'))) FROM (SELECT createddate FROM comments WHERE commentid = '1234');
SELECT TRUNC(MAX(createddatetm)) cdt FROM (SELECT TO_CHAR(createddate, 'Month dd, yyyy hh:mi AM') as createddatetm FROM comments WHERE commentid = '1234');
SELECT trunc(MAX(to_char(createddate, 'Month dd, yyyy hh:mi AM'))) FROM (SELECT createddate FROM comments WHERE commentid = '1234');

这是我的问题-
还有其他更简单的方法可以实现这一目标吗？或者我应该使用我提到的任何这些吗？此外，这些查询在性能方面是否存在差异？我认为没有，但我需要确认一次。

Answer 1

为什么要将日期转换为 varchar 以再次将其转换回 date/timestamp？您可以 compare/use 存储在数据库中的日期值。

您的问题可以使用 window 函数轻松解决：

select *
from ( 
  select c.*, 
         row_number() over (order by createddate desc) as rn
  from comments
  where commentid = 1234
) t
where rn = 1;

或者使用 max()

select *
from ( 
  select c.*, 
         max(createddate) over () as max_createddate
  from comments
  where commentid = 1234
) t
where max_createddate = createddate;

或者如果您需要多个 commentid 值：

select *
from ( 
  select c.*, 
         row_number() over (partition by commentid order by createddate desc) as rn
  from comments
) t
where rn = 1;

Answer 2

当您只是在寻找一个简单的值时，为什么要嵌套 Select 语句？

select  trunc( max( createddate )) cdt
from    comments
where   commentid = '1234';

在 SQL 中查找 MAX(DATE) 值

Finding MAX(DATE) value in SQL

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