分组以显示唯一字段

Question

目前，我的代码如下：

SELECT Volunteer.VolunteerID, Volunteer.LastName, Volunteer.FirstName, Observations.VolunteerID, Roost.RoostID, Roost.UTME, Roost.UTMN, Roost.DecimalDegrees, Observations.Date, Location.LocationName, Location.Address, Town.Town
FROM (((Volunteer INNER JOIN Observations ON Volunteer.[VolunteerID]= Observations.[VolunteeriD])
LEFT JOIN Roost ON Roost.[RoostID]= Observations.[RoostID])
LEFT JOIN Location ON Location.[LocationID]= Roost.[LocationID])
LEFT JOIN Town ON Location.[TownID]= Town.[TownID]
ORDER BY Volunteer.LastName, Volunteer.FirstName;

目前我的表格的一部分如下所示：

    Observations.VolunteerID | Observations.LocationID | Date

    Bob                      | Main St                 | 2015-01-01  
    Bob                      | Main St                 | 2015-02-02
    Sally                    | Fox St                  | 2015-02-02
    Dave                     | Long St                 | 2015-02-02
    Dave                     | Taylor St               | 2015-02-05
    Lindsay                  | New St                  | 2015-02-01
    Lindsay                  | New St                  | 2015-02-08
    Lindsay                  | Ray St                  | 2015-02-10
    Lindsay                  | Main St                 | 2015-02-25
    Lindsay                  | Taylor St               | 2015-02-31

但是，我想添加一个由 VolunteerID 和 LocationID 分组的 GROUP BY 语句（我认为），以便获得以下输出。我想知道志愿者去过的每个地点，如果他们多次访问同一个地点，我想查看最近的日期。我如何将它合并到我的原始代码中？

    Observations.VolunteerID | Observations.LocationID | Date
    Bob                      | Main St                 | 2015-02-02
    Sally                    | Fox St                  | 2015-02-02
    Dave                     | Long St                 | 2015-02-02
    Dave                     | Taylor St               | 2015-02-05
    Lindsay                  | New St                  | 2015-02-08
    Lindsay                  | Ray St                  | 2015-02-10
    Lindsay                  | Main St                 | 2015-02-25
    Lindsay                  | Taylor St               | 2015-02-31

---

更新了代码，但仍然出现语法错误（查询表达式“Volunteer.FirstName GROUP BY Observations.VolunteerID”中缺少运算符。

SELECT Volunteer.VolunteerID, Volunteer.LastName, Volunteer.FirstName, Observations.VolunteerID, Roost.RoostID, Roost.UTME, Roost.UTMN, Roost.DecimalDegrees, Max([Observations.Date]), Location.LocationName, Location.Address, Town.Town
FROM (((Volunteer INNER JOIN Observations ON Volunteer.[VolunteerID]= Observations.[VolunteeriD])
LEFT JOIN Roost ON Roost.[RoostID]= Observations.[RoostID])
LEFT JOIN Location ON Location.[LocationID]= Roost.[LocationID])
LEFT JOIN Town ON Location.[TownID]= Town.[TownID]
ORDER BY Volunteer.LastName, Volunteer.FirstName
GROUP BY Observations.VolunteerID, Observations.LocationID;

Answer 1

max 加上group by 子句似乎就足够了：

select Observations.VolunteerID, Observations.LocationID, max([Date])
from <...>
group by Observations.VolunteerID, Observations.LocationID

Answer 2

您需要在日期上使用GROUP BY 和聚合函数MAX()。

SELECT VolunteerID, LocationID, MAX(Date)
FROM(
SELECT 
  Volunteer.VolunteerID, 
  Volunteer.LastName, 
  Volunteer.FirstName, 
  Observations.VolunteerID, 
  Roost.RoostID, 
  Roost.UTME, 
  Roost.UTMN, 
  Roost.DecimalDegrees, 
  Observations.Date, 
  Location.LocationName, 
  Location.Address, 
  Town.Town
FROM 
  (((Volunteer 
  INNER JOIN Observations ON Volunteer.[VolunteerID]= Observations.[VolunteeriD])
  LEFT JOIN Roost ON Roost.[RoostID]= Observations.[RoostID])
  LEFT JOIN Location ON Location.[LocationID]= Roost.[LocationID])
  LEFT JOIN Town ON Location.[TownID]= Town.[TownID]
ORDER BY Volunteer.LastName, Volunteer.FirstName
) foo
GROUP BY VolunteerID, LocationID

Answer 3

将ORDER BY 子句放在GROUP BY 之后。

将Max([Observations.Date]) 更改为Max(Observations.Date)

将SELECT 列表中不是聚合表达式的任何字段表达式添加到您的GROUP BY。

SELECT
    Volunteer.VolunteerID,
    Volunteer.LastName,
    Volunteer.FirstName,
    Observations.VolunteerID,
    Roost.RoostID,
    Roost.UTME,
    Roost.UTMN,
    Roost.DecimalDegrees,
    Max(Observations.Date) AS latest_observation,
    Location.LocationName,
    Location.Address,
    Town.Town
FROM (((Volunteer INNER JOIN Observations ON Volunteer.[VolunteerID]= Observations.[VolunteeriD])
    LEFT JOIN Roost ON Roost.[RoostID]= Observations.[RoostID])
    LEFT JOIN Location ON Location.[LocationID]= Roost.[LocationID])
    LEFT JOIN Town ON Location.[TownID]= Town.[TownID]
GROUP BY
    Volunteer.VolunteerID,
    Volunteer.LastName,
    Volunteer.FirstName,
    Observations.VolunteerID,
    Roost.RoostID,
    Roost.UTME,
    Roost.UTMN,
    Roost.DecimalDegrees,
    Location.LocationName,
    Location.Address,
    Town.Town
ORDER BY Volunteer.LastName, Volunteer.FirstName;

但是我不确定该分组是否合适。在这个问题中，您想对 Observations.VolunteerID 和 Observations.LocationID 进行分组。这个较大的GROUP BY 可能不会给您相同的结果。

如果是这种情况，您可以对SELECT 字段使用聚合函数，然后从GROUP BY 中删除该字段。比如……

SELECT Max(Volunteer.LastName)

假设 LastName 对于给定的 VolunteerID 始终相同，Max(Volunteer.LastName) 将为您提供相同的值。