如何在LINQ中的子表上创建where条件

Question

这是对另一个问题的答案部分中的这个问题的回应.

我有一个Orders集合,每个Order订购OrderItems的集合,每个OrderItem都有一个PartId.使用LINQ如何实现以下SQL语句:

1)选择具有特定零件ID的所有订单

SELECT *
FROM Order
WHERE Id in (SELECT OrderId FROM OrderItems WHERE PartId = 100)

2)选择Order.OrderNumber和OrderItem.PartName

SELECT Order.OrderNumber, OrderItem.PartName
FROM Order INNER JOIN OrderItem ON Order.Id = OrderItem.OrderId
WHERE OrderItem.partId = 100

3)选择Order.OrderNumber和整个OrderItem细节:

SELECT Order.OrderNumber, OrderItem.*
FROM Order INNER JOIN OrderItem ON Order.Id = OrderItem.OrderId
WHERE OrderItem.partId = 100

Answer 1

实际代码应该是

1)

var orders = from o in Orders
             where o.OrderItems.Any(i => i.PartId == 100)
             select o;

Any()方法返回一个bool,就像SQL"in"子句一样.这将得到所有订单,其中任何OrderItems的PartId为100.

图2a)

// This will create a new type with the 2 details required 
var orderItemDetail = from o in Orders
                      from i in Orders.OrderItems
                      where i.PartId == 100
                      select new()
                      {
                          o.OrderNumber,
                          i.PartName
                      }

这两个from子句就像一个内连接.

图2b)

// This  will populate the OrderItemSummary type
var orderItemDetail = from o in Orders
                      from i in Orders.OrderItems
                      where i.PartId == 100
                      select new OrderItemSummary()
                      {
                          OriginalOrderNumber = o.OrderNumber,
                          PartName = i.PartName
                      }

3)

// This will create a new type with two properties, one being the
// whole OrderItem object.
var orderItemDetail = from o in Orders
                      from i in Orders.OrderItems
                      where i.PartId == 100
                      select new()
                      {
                          OrderNumber = o.OrderNumber,
                          Item = i
                       }

由于"i"是TypeItem类型的对象,因此Item被创建为OrderItem.