分组依据以显示独特的字段

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 table 的一部分看起来像这样：

    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 GROUP BY 语句（我认为），它由 VolunteerID 和 LocationID 分组，所以我得到以下输出。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。我如何将其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

已更新代码，但仍然出现语法错误（查询表达式中缺少运算符 'Volunteer.FirstName GROUP BY Observations.VolunteerID'。

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 和 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 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;

但是我不确定这种分组是否合适。在这个问题中，您想对 Observations.VolunteerID 和 Observations.LocationID 进行分组。这个更大的 GROUP BY 可能不会给你相同的结果。

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。例如...

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。

分组依据以显示独特的字段

Group by to show unique fields

sql

ms-access

group-by