LINQ - Left Join, Group By, and Count

Question

from p in context.ParentTable
join c in context.ChildTable on p.ParentId equals c.ChildParentId into j1
from j2 in j1.DefaultIfEmpty()
group j2 by p.ParentId into grouped
select new { ParentId = grouped.Key, Count = grouped.Count(t=>t.ChildId != null) }

Consider using a subquery:

from p in context.ParentTable 
let cCount =
(
  from c in context.ChildTable
  where p.ParentId == c.ChildParentId
  select c
).Count()
select new { ParentId = p.Key, Count = cCount } ;

---

If the query types are connected by an association, this simplifies to:

from p in context.ParentTable 
let cCount = p.Children.Count()
select new { ParentId = p.Key, Count = cCount } ;

LATE ANSWER:

You shouldn't need the left join at all if all you're doing is Count(). Note that join...into is actually translated to GroupJoin which returns groupings like new{parent,IEnumerable<child>} so you just need to call Count() on the group:

from p in context.ParentTable
join c in context.ChildTable on p.ParentId equals c.ChildParentId into g
select new { ParentId = p.Id, Count = g.Count() }

In Extension Method syntax a join into is equivalent to GroupJoin (while a join without an into is Join):

context.ParentTable
    .GroupJoin(
                   inner: context.ChildTable
        outerKeySelector: parent => parent.ParentId,
        innerKeySelector: child => child.ParentId,
          resultSelector: (parent, children) => new { parent.Id, Count = children.Count() }
    );

While the idea behind LINQ syntax is to emulate the SQL syntax, you shouldn't always think of directly translating your SQL code into LINQ. In this particular case, we don't need to do group into since join into is a group join itself.

Here's my solution:

from p in context.ParentTable
join c in context.ChildTable on p.ParentId equals c.ChildParentId into joined
select new { ParentId = p.ParentId, Count = joined.Count() }

Unlike the mostly voted solution here, we don't need j1, j2 and null checking in Count(t => t.ChildId != null)