SQL select max(date)和相应的值

Question

可能重复:
如何获取包含最大值的表的记录？

我有一个如下所示的聚合查询:

SELECT TrainingID, Max(CompletedDate) as CompletedDate, Max(Notes) as Notes     --This will only return the longest notes entry
FROM HR_EmployeeTrainings ET
WHERE (ET.AvantiRecID IS NULL OR ET.AvantiRecID = @avantiRecID)
GROUP BY AvantiRecID, TrainingID            

哪个有效,并且大部分时间都返回正确的数据,但我注意到了一个问题.返回的Notes字段不一定与max(completedDate)来自的记录匹配.相反,它将是最长的字符串？或者具有最高ASCII值的那个？如果两个记录之间存在关联,SQL Server会做什么？我甚至都不确定.我想得到的是max(completedDate)记录中的notes字段.我应该怎么做呢？

Answer 1

您可以使用子查询.子查询将获得Max(CompletedDate).然后,您可以获取此值并再次加入您的表格以检索与该日期关联的备注:

select ET1.TrainingID,
  ET1.CompletedDate,
  ET1.Notes
from HR_EmployeeTrainings ET1
inner join
(
  select Max(CompletedDate) CompletedDate, TrainingID
  from HR_EmployeeTrainings
  --where AvantiRecID IS NULL OR AvantiRecID = @avantiRecID
  group by TrainingID
) ET2
  on ET1.TrainingID = ET2.TrainingID
  and ET1.CompletedDate = ET2.CompletedDate
where ET1.AvantiRecID IS NULL OR ET1.AvantiRecID = @avantiRecID

Answer 2

是啊,这就是它在SQL中的意图.你可以单独获得每列的最大值.您似乎希望从具有最大日期的行返回值,因此您必须选择具有最大日期的行.我更喜欢使用subselect来执行此操作,因为查询保持紧凑易读.

SELECT TrainingID, CompletedDate, Notes
FROM HR_EmployeeTrainings ET 
WHERE (ET.AvantiRecID IS NULL OR ET.AvantiRecID = @avantiRecID) 
AND CompletedDate in 
   (Select Max(CompletedDate) from HR_EmployeeTrainings B
    where B.TrainingID = ET.TrainingID)

如果您还想通过AntiRecID进行匹配,那么您也应该在subselect中包含它.