多个表上的 Sql Joins 返回两列的乘积答案

【问题标题】：Sql Joins on multiple table returning product of two columns多个表上的 Sql Joins 返回两列的乘积
【发布时间】：2016-05-05 13:38:00
【问题描述】：

我正在尝试在 asp.net 中的 sql server 数据库上生成报告，并且我将某些列的结果作为两列的乘积。这是代码

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

当我运行代码时，会显示结果，但“CourseCount”和“StudentCount”列显示的值是每个单独列的乘积。 “CourseCount”通常是 288，“StudentCount”是 38，但是当我运行代码时，“CourseCount”和“StudentCount”都显示 10944，即 38 x 288。

无论如何我可以让它们显示正确的值？

【问题讨论】：

标签： sql asp.net sql-server database

【解决方案1】：

将您的代码从使用所有行的计数更改为仅使用不同值的计数，应该会返回您期望的结果

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

如果所有学校都有课程，并且所有课程都有学生，则返回的结果在技术上是正确的

【讨论】：

工作得很好！非常感谢@rossquan。

【解决方案2】：

如上所述，这就是您使用 COUNT() 的方式，您要求它计数所有，这就是它返回这么多的原因。仅对要计算的两个值使用 count。

【讨论】：

【解决方案3】：

这可能比 DISTINCT Count 方法执行得更好..

comm.CommandText = 
    "SELECT cc.CourseCount, sc.StudentCount, Schools.Name, Schools.StartDate, Schools.SchoolFees " +
    "FROM Schools" +
    "OUTER APPLY (SELECT Count(Students.StudentID) StudentCount FROM Students WHERE Students.SchoolID = Schools.SchoolID) sc " +
    "OUTER APPLY (SELECT Count(Courses.CourseID) CourseCount FROM Courses WHERE Courses.SchoolID = Schools.SchoolID) cc " +
    "WHERE Schools.Active = 1 ";

【讨论】：