c# - Group by in Linq to Sql -

i'm trying write linq query:

sql:

select s.s_name, sum(sub.evaluation) suma submit_task sub join student s on s.id=sub.student_id join study_group g on g.id=s.study_group_id g.g_name="abcd" group s.s_name order suma desc 

var groupname = "abcd"; var query =     submittask in db.submit_task     join student in db.student         on submittask.student_id equals student.id     join studygroup in db.study_group         on student.study_group_id equals studygroup.id     studygroup.g_name == groupname     group submittask student.s_name g     let suma = g.sum(st => st.evaluation)     orderby suma descending     select new     {         s_name = g.key,         suma = suma,     };