Question

I have this query:

(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)

This returns:

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

However I’d like it to return this:

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

UPDATE:
This is what Students, Classes and SickDays look like using @sixlettervariables example:

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 },
};

Ok the above is now correct, so sorry for the confusion!

Answer 1

This is known as a Left Outer Join. In this case, every Student is required even if they have no Sick Days. So, we would start with the students, perform a left outer join with the sick days, and then return the count of the classes which matched the ID you had specified:

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)
        };