LINQ - Left Join, Group By, and Count


Question

Let's say I have this SQL:

SELECT p.ParentId, COUNT(c.ChildId)
FROM ParentTable p
  LEFT OUTER JOIN ChildTable c ON p.ParentId = c.ChildParentId
GROUP BY p.ParentId

How can I translate this into LINQ to SQL? I got stuck at the COUNT(c.ChildId), the generated SQL always seems to output COUNT(*). Here's what I got so far:

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() }

Thank you!

1
165
3/29/2009 10:42:05 PM

Accepted Answer

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) }
185
3/29/2009 10:28:29 PM

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 } ;

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