MS Access SQL找到一个值第一次出现

Question

嗨我想在物种数据库上运行查询。我想查询独特的值，但也发现物种第一次被发现的那一年。

到目前为止，我有这样的：

SELECT DISTINCT [Genus_HeTR] & " " & [Species_HeTR] AS Species 
FROM HeTR_Rec 
WHERE [Species_HeTR] <> "sp." 
UNION SELECT DISTINCT [Genus_HeOP] & " " & [Species_HeOP] AS Species 
FROM HeOP_Rec 
WHERE [Species_HeOP] <> "" 
AND [Species_HeOP] <> "sp."; 

我串接属和种的名称，并从两个不同的表添加数据（因此UNION）。这提供了一个物种清单，但我想知道在这个地方看到物种的一年。

Answer 1

我会冒险猜测你的源表格包含一个日期/时间字段，其中存储了每个观察日期。如果是这样，您可以从2个表中获取UNION数据，并将其用作GROUP BY查询中的子查询源，从中获取每个物种的最小观测年份。

SELECT 
    sub.Species, 
    Min(sub.observation_year) AS first_sighting_year 
FROM 
    (
     SELECT 
      [Genus_HeTR] & " " & [Species_HeTR] AS Species, 
      Year(observation_date) AS observation_year 
     FROM HeTR_Rec 
     WHERE [Species_HeTR] <> "sp." 
     UNION ALL 
     SELECT 
      [Genus_HeOP] & " " & [Species_HeOP] AS Species, 
      Year(observation_date) AS observation_year 
     FROM HeOP_Rec 
     WHERE [Species_HeOP] <> "" 
     AND [Species_HeOP] <> "sp." 
    ) AS sub 
GROUP BY sub.Species;