在 SQL 中查找 MAX(DATE) 值

Question

我是 SQL 新手，所以这个问题-

我的表中有一个DATE 列，我的查询应该使用最新的DATE 获取值。我意识到我可以通过以下任何一种方式来实现它-

1. 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');

2. 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');

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

4. 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-
有没有其他更简单的方法可以实现这一目标？或者我应该使用我已经提到的任何这些？另外，这些查询在性能方面有什么区别吗？我不认为有，但我需要确认一次。

Answer 1

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';

Answer 2

为什么要将日期转换为 varchar 以再次将其转换回日期/时间戳？您可以只比较/使用存储在数据库中的日期值。

使用窗口函数可以轻松解决您的问题：

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;