Question

我正在尝试在asp.net中生成一个sql server数据库的报告，我将一些列的结果作为两列的产品。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，显示结果，但“CourseCount”和“StudentCount”的列显示的值是每个列的乘积。 “CourseCount”通常为288，“StudentCount”为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吗？

Answer 1

将代码从使用所有行的计数更改为仅包含不同值的计数，应该返回您期望的结果

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

，那么返回的结果在技术上是正确的

Answer 2

如上所述，你是如何使用COUNT（），你要求它全部计算，这就是为什么它返回这么多。仅对要计算的两个值使用count。

Answer 3

这可能比DISTINCT Count方法表现更好。

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 ";

Sql连接多个表返回两列的产品

3 个答案: