SELECT COUNT（DISTINCT [名]）

Question

我可以從列在一個表像這樣執行不同的（或者非重複的名字）下面的SQL Server的選擇：

SELECT COUNT(DISTINCT [Name]) FROM [MyTable] 

但如果我有多個表（所有這些表都包含名爲[Name]的名稱字段），我需要知道兩個或多個表中非重複名稱的計數。

如果我運行是這樣的：

SELECT COUNT(DISTINCT [Name]) FROM [MyTable1], [MyTable2], [MyTable3] 

我得到一個錯誤，「不明確的列名‘名稱’」。

PS。所有三個表格[MyTable1]，[MyTable2]，[MyTable3]都是之前選擇的產品。

Answer 1

澄清後，使用：

SELECT x.name, COUNT(x.[name]) 
    FROM (SELECT [name] 
      FROM [MyTable] 
      UNION ALL 
      SELECT [name] 
      FROM [MyTable2] 
      UNION ALL 
      SELECT [name] 
      FROM [MyTable3]) x 
GROUP BY x.name 

如果我理解正確，使用：

SELECT x.name, COUNT(DISTINCT x.[name]) 
    FROM (SELECT [name] 
      FROM [MyTable] 
      UNION ALL 
      SELECT [name] 
      FROM [MyTable2] 
      UNION ALL 
      SELECT [name] 
      FROM [MyTable3]) x 
GROUP BY x.name 

UNION將刪除重複; UNION ALL不會，而且速度更快。

Answer 2

編輯：看到最近的評論後不得不改變。

這是否給你你想要的？在合併來自所有表格的行之後，這給每個人一個計數。

SELECT [NAME], COUNT(*) as TheCount 
FROM 
    (
    SELECT [Name] FROM [MyTable1] 
    UNION ALL 
    SELECT [Name] FROM [MyTable2] 
    UNION ALL 
    SELECT [Name] FROM [MyTable3] 
    ) AS [TheNames] 
GROUP BY [NAME] 

Answer 3

這裏的另一種方式：

SELECT x.name, SUM(x.cnt) 
FROM (SELECT [name], COUNT(*) AS cnt 
     FROM [MyTable] 
     GROUP BY [name] 
    UNION ALL 
     SELECT [name], COUNT(*) AS cnt 
     FROM [MyTable2] 
     GROUP BY [name] 
    UNION ALL 
     SELECT [name], COUNT(*) AS cnt 
     FROM [MyTable3] 
     GROUP BY [name] 
    ) AS x 
GROUP BY x.name