是否需要轉換SQL到LINQ幫助 - LEFT JOIN以計數

Question

我需要幫助下面的SQL查詢轉換爲LINQ：

select s.teacherid,t.lastname,t.firstname,t.title,t.grade, count(s.TeacherID) 
    from Teacher t 
    left join Student s on t.teacherid = s.teacherid 
    and t.isactive = 1 
    and s.isactive = 1 
    group by s.TeacherID,t.lastname,t.firstname,t.title,t.grade 

---

I tried the following but it is not returning teacher records who have no students: 

    var teachers = 
     (from t in Teachers 
     join s in Students on t.TeacherID equals s.TeacherID 
     into results 
     where t.IsActive == true 
     from r in results          
     group r by new { r.TeacherID, r.Teacher.FirstName, r.Teacher.LastName, r.Teacher.Title, r.Teacher.Grade} into g 
     select new { TeacherID = g.Key.TeacherID,FirstName = g.Key.FirstName, LastName = g.Key.LastName, Title=g.Key.Title, Grade = g.Key.Grade}     
     ); 

預先感謝您！

Answer 1

我做了一些研究，並找到了解決辦法。下面的表達式固定它：

var results = Teachers 
       .Where(t => t.IsActive == true) 
       .Select(t => 
       { 
        TeacherID = t.TeacherID, 
        FirstName = t.FirstName, 
        LastName = t.LastName, 
        Title = t.Title, 
        Grade = t.Grade, 
        Count = t.Students.Where(s => s.IsActive == true).Count()     
       }); 
results.ToList().Dump(); 

Answer 2

爲了做一個左連接，你需要使用DefaultIfEmpty()方法，像這樣：

from r in results.DefaultIfEmpty() 

Answer 3

var teachers = 
    (from t in Teachers 
    join s in Students on t.TeacherID equals s.TeacherID 
    into results 
    where t.IsActive == true 
    from r in results.DefaultIfEmpty()          
    group r by new { r.TeacherID, r.Teacher.FirstName, r.Teacher.LastName, r.Teacher.Title, r.Teacher.Grade} into g 
    select new { TeacherID = g.Key.TeacherID,FirstName = g.Key.FirstName, LastName = g.Key.LastName, Title=g.Key.Title, Grade = g.Key.Grade}     
    );