使用子连接和子查询的SQL to LINQ

SELECT *
FROM Orders
WHERE CustomerID IN (SELECT CustomerID FROM Customers WHERE Country = 'USA')

var query = from o in db.Orders
            where (from c in db.Customers
                   where c.Country == "USA"
                   select c.CustomerID).Contains(o.CustomerID)
            select o;

SELECT OrderID, OrderDate, (SELECT COUNT(*) FROM OrderDetails WHERE OrderID = o.OrderID) AS TotalItems
FROM Orders o

var query = from o in db.Orders
            select new
            {
                o.OrderID,
                o.OrderDate,
                TotalItems = (from od in db.OrderDetails
                              where od.OrderID == o.OrderID
                              select od).Count()
            };