Linq to SQL-即使0時也要獲得count（）的查詢

Question

我有這個查詢：

(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)

返回：

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

但是我希望它返回此：

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

更新：這是使用@sixlettervariables示例的學生，班級和病假的外觀：

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 },
};

好的，以上內容現在是正確的，對於造成的混亂，我們深表歉意！

Answer 1

這稱為左外部聯接 。 在這種情況下，即使沒有病假 ，也需要每個學生 。 因此，我們將從學生開始，在病假期間進行左外連接，然后返回與您指定的ID相匹配的班級數量：

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，即使他們不在班上，您也希望所有的學生嗎？ 我認為您需要左加入。 與Linq這可以使用DefaultIfEmpty（）實現

嘗試這個？

(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)

我希望這能使您朝正確的方向發展。