I can not understand the Group Join in Linq

Question

I have already searched on the forums and found a diagram.

 

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LINQ supports grouping joins which are a superset of inner and left outer join. A grouping join produces a hierarchical result that pairs each element from the outer sequence with its corresponding sequence of elements from the inner sequence. For example (from the Standard Query Operators documentation):

 

var custTotalOrders = from c in customers join o in orders on c.CustomerID equals o.CustomerID into co select new { c.Name, TotalOrders = co.Sum(o => o.Total) };
can be written as a grouping join followed by an iteration of the grouped orders

 

var custTotalOrders = from c in customers join o in orders on c.CustomerID equals o.CustomerID into co from o in co select new { c.Name, o.Total };
The query can be turned into a left outer join by applying the DefaultIfEmpty operator to the grouped orders

 

var custTotalOrders = from c in customers join o in orders on c.CustomerID equals o.CustomerID into co from o in co.DefaultIfEmpty() select new { c.Name, Total = o != null ? o.Total : 0 };
DefaultIfEmpty() turns an empty sequence into a sequence with a single null element. Thus, the code must now be prepared to deal with a null o.
Hope this helps. For more details, check out the Standard Query Operators documentation (in the Docs directory under your LINQ preview install) section 1.6.2.
Anders

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But I still can not understand the group join,

 

i.e.

 

Code Block

var query = from customerRow in db.CustomerTable
            join orderRow in db.OrderTable
                on customerRow.CustomerID equals orderRow.CustomerID into customerOrders
            from orderRow in customerOrders.DefaultIfEmpty()
            select new
            {
                customerRow.CustomerID,
                OrderID = orderRow.OrderID == null ? -1L : orderRow.OrderID
            };

 

 

what is the customerOrders here?

and I do not understand from orderRow in customerOrders.DefaultIfEmpty() , I think if the orderRow is empty, the DefaultIfEmpty() should return null, so the empty order rows shoule not be included in the result, but it is there.

 

So what is the customerOrders indeed? what hierarchical structure it is?

 

 

I have two tables:

CustomerTable

( CustomerID | CustomerName)

   1                   John

   2                   Jerry

   3                   Joke

   4                   Jenny

 

OrderTable

( OrderID | CustomerID )

   1                    1

   2                    1

   3                    1

   4                    2

 

 

what is the group join result of these tables?

 

Thanks