How to do a full outer join in Linq?

Question

I've inherited a database that wasn't designed exactly optimally, and I need to manipulate some data. Let me give a more common analogy of the kind of thing I have to do:

Let's say we have a Student table, a StudentClass table keeping record of all the classes he attended, and a StudentTeacher table that stores all the teachers who taught this student. Yes, I know it's a dumb design and it would make more sense to store the teacher on the Class table - but that's what we're working with.

I now want to clean up the data, and I want to find all the places where a student has a teacher but no classes, or a class but no teachers. SQL thus:

select *
from StudentClass sc
full outer join StudentTeacher st on st.StudentID = sc.StudentID
where st.id is null or sc.id is null

How do you do that in Linq?

Answer 1

A start...

 var q = from sc in StudentClass
            join st in StudentTeachers on sc.StudentID equals st.StudentID into g
            from st in g.DefaultIfEmpty()
            select new {StudentID = sc.StudentID, StudentIDParent = st == null ? "(no StudentTeacher)" : st.StudentID...........};

See also http://www.linqpad.net/ for more samples Good tool to play with

Answer 2

for the given 2 collections a and b, a required full outer join might be as following:

a.Union(b).Except(a.Intersect(b));

If a and b are not of the same type, then 2 separate left outer joins are required:

var studentsWithoutTeachers =
    from sc in studentClasses
    join st in studentTeachers on sc.StudentId equals st.StudentId into g
    from st in g.DefaultIfEmpty()
    where st == null
    select sc;
var teachersWithoutStudents =
    from st in studentTeachers
    join sc in studentClasses on st.StudentId equals sc.StudentId into g
    from sc in g.DefaultIfEmpty()
    where sc == null
    select st;

here is a one line option using Concat():

(from l in left
 join r in right on l.Id equals r.Id into g
 from r in g.DefaultIfEmpty()
 where r == null
 select new {l, r})
     .Concat(
     from r in right
     join sc in left on r.Id equals sc.Id into g
     from l in g.DefaultIfEmpty()
     where l == null
     select new {l, r});

Answer 3

I think I have the answer here, which is not as elegant as I'd hoped, but it should do the trick:

var studentIDs = StudentClasses.Select(sc => sc.StudentID)
  .Union(StudentTeachers.Select(st => st.StudentID)
  .Distinct();
var q =
  from id in studentIDs
  join sc in StudentClasses on id equals sc.StudentID into jsc
  from sc in jsc.DefaultIfEmpty()
  join st in StudentTeachers on id equals st.StudentID into jst
  from st in jst.DefaultIfEmpty()
  where st == null ^ sc == null
  select new { sc, st };

You could probably squeeze these two statements into one, but I think you'd sacrifice code clarity.