在SQL中查找MAX(DATE)值

Question

我是SQL的新手,因此这个问题 -

DATE我的表中有一列,我的查询应该用最新的值获取值DATE.我意识到我可以通过以下任何方式实现 -

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？或者我应该使用我提到过的任何一种吗？此外,这些查询在性能方面是否存在差异？我不认为有,但我需要确认一次.

Answer 1

为什么要将日期转换为varchar以将其再次转换回日期/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;

或者如果你需要多个有价值的价值:

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