在表中使用COUNT函數加入

我正在爲連接三個表的報表頁編寫sql代碼。這是我寫的查詢。在表中使用COUNT函數加入

comm.CommandText = "SELECT Count(DISTINCT Courses.CourseID) AS CourseCount, Count(DISTINCT Students.StudentID) AS StudentCount, Count(Students.StartDate) AS StartCount, School.Name, School.StartDate, School.SchoolFees " + 
       "FROM Schools " + 
       "LEFT JOIN Courses ON (School.SchoolID = Courses.SchoolId) " + 
       "LEFT JOIN Students ON (School.SchoolID = Student.SchoolID) " + 
       "WHERE School.Active = 1 " + 
       "GROUP BY School.Name, School.StartDate, School.SchoolFees";

以上查詢效果很好。但是我想顯示Student.StartDate滿足條件的每個School的Student.StartDate的計數。下面是我想用

SELECT Count(Students.StartDate) 
FROM Students 
WHERE Student.StartDate >= DATEADD(month, -1, GETDATE());

我想上面的查詢被返回作爲我的主要查詢的一部分，但不知道如何實現查詢。任何幫助將不勝感激。謝謝

來源

2016-05-09 elfico

當你想從不同的表彙總信息，您應該不加入表格然後進行彙總，但始終先建立彙總，然後加入這些彙總。在你的情況下，你可以通過計算不同的ID來避免問題，但這並不總是可能的（即當尋找總數或avarages時）。你可以有條件地計數CASE WHEN。

SELECT 
    COALESCE(c.CourseCount, 0) AS CourseCount, 
    COALESCE(s.StudentCount, 0) AS StudentCount, 
    COALESCE(s.StartCount, 0) AS StartCount, 
    School.Name, 
    School.StartDate, 
    School.SchoolFees 
FROM Schools 
LEFT JOIN 
(
    SELECT SchoolID, COUNT(*) AS CourseCount 
    FROM Courses 
    GROUP BY SchoolID 
) c ON c.SchoolId = School.SchoolID 
LEFT JOIN 
(
    SELECT 
    SchoolID, 
    COUNT(*) AS StudentCount, 
    COUNT(CASE WHEN StartDate >= DATEADD(month, -1, GETDATE() THEN 1 END) as StartCount 
    FROM Students 
    GROUP BY SchoolID 
) s ON s.SchoolId = School.SchoolID 
WHERE School.Active = 1;

在情況下，它保證每所學校至少有一個學生，一個療程（這可能是這種情況），你可以改變外部聯接爲內部連接，並得到從而擺脫COALESCE表達式。

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2016-05-09 10:49:17

您的回答非常有用和信息豐富。我學到了一些新東西。非常感謝。我有一個小問題。當擺脫COALESCE表達式，這是否意味着我只有「SELECT CourseCount，StudentCount .....」。 – elfico

@elfico：確實。從不同表格中選擇數據時，我使用表格限定符來顯示列的起始位置 - 即使列名稱應足夠描述：「SELECT c.CourseCount，s.StudentCount，s.StartCount ...」 –

你可以用條件聚合來做到這一點。只需添加這到SELECT：

SUM(CASE WHEN Student.StartDate >= DATEADD(month,-1, GETDATE()) THEN 1 ELSE 0 END) as RecentStudents

來源

2016-05-09 10:31:59

謝謝你的回答。我試了一下，它的工作，但總和不到1.我期待21，但得到20。@Kettner的答案，並得到正確的答案。學到了一些新思想。謝謝 – elfico

使用CASE WHEN，就可以解決這個問題，

SELECT Count(DISTINCT Courses.CourseID) AS CourseCount, 
     Count(DISTINCT Students.StudentID) AS StudentCount, 
     Count(Students.StartDate) AS StartCount, 
     Sum(CASE WHEN Student.StartDate >= DATEADD(month,-1, GETDATE()) 
       THEN 1 ELSE 0 END) AS StartDateCount , 
     School.Name, School.StartDate, School.SchoolFees 
FROM Schools 
LEFT JOIN Courses ON (School.SchoolID = Courses.SchoolId) 
LEFT JOIN Students ON (School.SchoolID = Student.SchoolID) 
WHERE School.Active = 1 
GROUP BY School.Name, School.StartDate, School.SchoolFees

來源

2016-05-09 10:32:29

謝謝你的回答。我試了一下，它的工作，但總和不到1.我期待21，但得到20。@Kettner的答案，並得到正確的答案。學到了一些新思想。謝謝。 – elfico

我想這有助於

myConnect.Open(); 
     comm.CommandText = "SELECT Count(DISTINCT Courses.CourseID) AS CourseCount, Count(DISTINCT Students.StudentID) AS StudentCount, Count(Students.StartDate) AS StartCount, School.Name, School.StartDate, School.SchoolFees, " + 
     "(SELECT Count(stu.StartDate) FROM Students stu WHERE School.SchoolID = stu.SchoolID AND stu.StartDate >= DATEADD(month,-1, GETDATE())) AS CountStartDate" + // your count 
     "FROM Schools " + 
     "LEFT JOIN Courses ON (School.SchoolID = Courses.SchoolId) " + 
     "LEFT JOIN Students ON (School.SchoolID = Student.SchoolID) " + 
     "WHERE School.Active = 1 " + 
     "GROUP BY School.Name, School.StartDate, School.SchoolFees";

來源

2016-05-09 10:38:07

它沒有工作。我使用@kettner的答案，它的工作。感謝您的幫助。 – elfico

在表中使用COUNT函數加入

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