MS Access SQL找到一個值第一次出現

Question

嗨我想在物種數據庫上運行查詢。我想查詢獨特的值，但也發現物種第一次被發現的那一年。

到目前爲止，我有這樣的：

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."; 

我串接屬和種的名稱，並從兩個不同的表添加數據（因此UNION）。這提供了一個物種清單，但我想知道在這個地方看到物種的一年。

Answer 1

我會冒險猜測你的源表格包含一個日期/時間字段，其中存儲了每個觀察日期。如果是這樣，您可以從2個表中獲取UNION數據，並將其用作GROUP BY查詢中的子查詢源，從中獲取每個物種的最小觀測年份。

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;