Question

可能重复：
How to get the record of a table who contains the maximum value?

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，您再次获取此值并再次加入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

啊，是的，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)

如果您还希望通过AntiRecID匹配，也应该在子选择中包含该内容。

Answer 3

没有简单的方法可以做到这一点，但这样的事情会起作用：

SELECT ET.TrainingID, 
  ET.CompletedDate, 
  ET.Notes
FROM 
HR_EmployeeTrainings ET
inner join
(
  select TrainingID, Max(CompletedDate) as CompletedDate
  FROM HR_EmployeeTrainings
  WHERE (ET.AvantiRecID IS NULL OR ET.AvantiRecID = @avantiRecID)
  GROUP BY AvantiRecID, TrainingID  
) ET2 
  on ET.TrainingID = ET2.TrainingID
  and ET.CompletedDate = ET2.CompletedDate

Answer 4

单独评估每个MAX功能。因此MAX（CompletedDate）将返回最新CompletedDate列的值，MAX（Notes）将返回最大值（即字母最高值）。

您需要以不同方式构建查询以获得所需内容。这个问题实际上已经被多次询问和回答，所以我不再重复了：

How to find the record in a table that contains the maximum value?

Finding the record with maximum value in SQL

SQL select max（date）和相应的值

4 个答案: