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How to: Handle Composite Keys in Queries

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Some operators can take only one argument. If your argument must include more than one column from the database, you must create an anonymous type to represent the combination.

Example 1

The following example shows a query that invokes the GroupBy operator, which can take only one key argument.

        var query =
from cust in db.Customers
group cust.ContactName by new { City = cust.City, Region = cust.Region };

        foreach (var grp in query)
        {
            Console.WriteLine("\nLocation Key: {0}", grp.Key);
            foreach (var listing in grp)
            {
                Console.WriteLine("\t{0}", listing);
            }
        }

Dim query = _
From cust In db.Customers _
Group cust.ContactName By Key = New With {cust.City, cust.Region} _
Into Group

For Each grp In query
    Console.WriteLine("Location Key: {0}", grp.Key)
    For Each listing In grp.Group
        Console.WriteLine(vbTab & "0}", listing)
    Next
Next

Example 2

The same situation pertains to joins, as in the following example:

        var query =
from ord in db.Orders
from prod in db.Products
join det in db.OrderDetails
    on new { ord.OrderID, prod.ProductID } equals new { det.OrderID, det.ProductID }
    into details
from det in details
select new { ord.OrderID, prod.ProductID, det.UnitPrice };

Dim query = From ord In db.Orders, prod In db.Products _
            Join det In db.OrderDetails _
            On New With {ord.OrderID, prod.ProductID} Equals _
            New With {det.OrderID, det.ProductID} _
            Select ord.OrderID, prod.ProductID, det.UnitPrice

See also