如何:处理查询中的复合键

有些运算符只能带一个自变量。 如果您的参数必须包含数据库中的多个列,则您必须创建一个匿名类型来表示这种组合。

示例 1

下面的示例显示了调用 GroupBy 运算符的查询,此运算符只能带一个 key 自变量。

        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

示例 2

联接也属于这种情况,如下例中所示:

        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

请参阅