LINQ到SQL - 查询获得计数（），即使0

Question

我有这个疑问：

(from a in SickDays 
    join b in Class.Where(p => p.ID == myId) on a.Class_ID equals b.ID 
    join c in Student on a.Student_ID equals c.ID 
    group a by new { c.Name, c.Order } into ac 
    select new { Count = ac.Count(), Name = ac.Key.Name, Order = ac.Key.Order } 
).OrderBy(f => f.Order) 

这将返回：

Count | Name | Order 
    3 | Dave |  a 
    2 | John |  b 
    7 | Sally|  c 

但是我想它返回此：

Count | Name | Order 
    3 | Dave |  a 
    2 | John |  b 
    7 | Sally|  c 
    0 | Mark |  d 
    0 | Betty|  e 

UPDATE： 这就是学生，类和SickDays看喜欢用@sixlettervariables例如：

var Students = new List<Student>() 
{ 
    new Student { Id = 1, Name = "Al", Order = 'a' }, 
    new Student { Id = 2, Name = "Betty", Order = 'b' }, 
    new Student { Id = 3, Name = "Charles", Order = 'c' }, 
}; 

var Classes = new List<Class>() 
{ 
    new Class { Id = 1, Title = "A100" }, 
    new Class { Id = 2, Title = "A200" }, 
}; 

var SickDays = new List<SickDay>() 
{ 
    new SickDay { Id = 1, StudentId = 1, ClassId = 1 }, 
    new SickDay { Id = 2, StudentId = 1, ClassId = 1 }, 
    new SickDay { Id = 3, StudentId = 1, ClassId = 2 }, 
    new SickDay { Id = 4, StudentId = 1, ClassId = 2 }, 
    new SickDay { Id = 5, StudentId = 2, ClassId = 1 }, 
}; 

好上面现在是正确的，对于这样的困惑对不起！

Answer 1

这就是所谓的Left Outer Join。在这种情况下，即使他们没有生病日，也需要每个学生。所以，我们会与学生开始，进行左外与病假加入，然后返回您指定了其相匹配的ID类的计数：

var query = 
    from s in Students 
    join d in SickDays on s.ID equals d.Student_ID into gj 
    from sd in gj.DefaultIfEmpty() 
    group sd by new { s.Name, s.Order } into gg 
    select 
     new 
     { 
      Name = gg.Key.Name, 
      Order = gg.Key.Order, 
      Count = gg.Count(x => x != null && x.Class_ID == myId) 
     }; 

Answer 2

所以你想要所有的学生，即使他们不在课堂上？那么你需要一个我相信的左连接。用Linq这个是用DefaultIfEmpty（）实现的

试试这个吗？

(from a in SickDays 
    join b in Class.Where(p => p.ID == myId) on a.Class_ID equals b.ID 
    join c in Student on a.Student_ID equals c.ID 
     into s 
    from students in s.DefaultIfEmpty() 
    group a by new { students.Name, students.Order } into ac 
    select new { Count = ac.Count(), Name = ac.Key.Name, Order = ac.Key.Order } 
).OrderBy(f => f.Order) 

我希望能让你走向正确的方向。