在sql中計數匹配

Question

我想跟蹤工作中不同部門進行了多少次調查。 通過使用「分組依據」並計算匹配數量，這相當容易。

SELECT  Departments.DepartmentName, count(*) as [survey count] 
FROM   Departments INNER JOIN 
        Surveys ON Departments.DepartmentID = Surveys.DepartmentID 
GROUP BY Departments.DepartmentName 

但這隻顯示已完成調查的部門。我如何讓未完成調查的部門在結果清單中顯示爲零計數？

更新：

SELECT  Departments.DepartmentName AS Department, 
     COUNT(Surveys.DepartmentID) AS  [survey count] 
     , Departments.DepartmentID 
     FROM   Surveys FULL OUTER JOIN 
        Departments ON Surveys.DepartmentID = Departments.DepartmentID 
    GROUP BY Departments.DepartmentName, Departments.DepartmentID 

Answer 1

只需將您的INNER JOIN更改爲LEFT JOIN，然後將COUNT（）更改爲僅引用SURVEYS。那可能會如此。

SELECT  Departments.DepartmentName, 
      count(Surveys.DepartmentID) as [survey count] 
FROM   Departments LEFT JOIN 
       Surveys ON Departments.DepartmentID = Surveys.DepartmentID 
GROUP BY Departments.DepartmentName 

Answer 2

更換INNER JOIN與LEFT JOIN。它將選擇所有部門，即使沒有相關的調查存在。

Answer 3

SELECT Departments.DepartmentName, count(Surveys.DepartmentID) as [survey count] 
FROM Departments 
LEFT OUTER JOIN Surveys 
ON Departments.DepartmentID = Surveys.DepartmentID 
GROUP BY Departments.DepartmentName 

Answer 4

SELECT  Departments.DepartmentName, count(Surveys.*) as [survey count] 
FROM  Departments 
LEFT JOIN Surveys ON Departments.DepartmentID = Surveys.DepartmentID 
GROUP BY Departments.DepartmentName 

注意在LEFT變化JOIN和count（調查。*）

Answer 5

你需要一個 「左外連接」，而不是 「內部連接」 - 將在輸出中將所有與「調查」對應的列生成爲null，其中沒有匹配連接條件。

您需要將您的查詢更新爲「count（Surveys.DepartmentID）」或「sum（如果surveys.departmentID不爲null，則爲1 else 0 end）」，以避免將這些行統計爲單個提交。