将详细表行转换为linq到sql lambda表达式主详细关系

如何解决将详细表行转换为linq到sql lambda表达式主详细关系

假设我们具有如下的主从关系

**Order Table**
   OrderId
   OrderDate

**OrderItem Table**
   OrderItemId
   OrderId 
   ItemId1
   Price1
   ItemId2
   Price2
   ItemId3
   Price3


**Item Table**
    ItemId
    ItemName 

每个订单最多可以包含3个订单项。因此，OrderItem表的设计如上所述。如何编写提供以下输出的LINQ to SQL lambda表达式？

OrderId -- OrderDate -- Item1Name -- Item1Price -- Item2Name -- Item2Price -- Item3Name -- Item3Price
 1          8/27/2020    Item1        30            Item2        40
 2          8/27/2020    Item1        20            Item5        15            Item8        40
 3          8/27/2020    Item4        30

解决方法

下面的解决方案不是最好的解决方案，如果我们知道更多有关您使用的是哪种技术（.NET或.NETCore）的信息，可以对其进行改进，因此请回答上述问题，我们将为您提供更多帮助。目前，这是我能为您做的最好的事情：

var resultSet = (from orderItem in dataContext.OrderItem
                join order in dataContext.Order on orderItem.OrderId equals order.OrderId
                let firstItem = (from item in dataContext.Item
                                 where item.ItemId == orderItem.ItemId1
                                 select item).FirstOrDefault()
                let secondItem = (from item in dataContext.Item
                                 where item.ItemId == orderItem.ItemId2
                                 select item).FirstOrDefault()
                let thirdItem = (from item in dataContext.Item
                                 where item.ItemId == orderItem.ItemId3
                                 select item).FirstOrDefault()
                let firstItemExists = firstItem != null
                let secondItemExists = secondItem != null
                let thirdItemExists = thirdItem != null
                select new 
                {
                    OrderId = order.OrderId
                    OrderDate = order.OrderDate,Item1Name = firstItemExists
                                ? firstItem.ItemName
                                : string.Empty
                    Item1Price = firstItemExists
                                ? firstItem.ItemPrice
                                : null
                    Item2Name = secondItemExists
                                ? secondItem.ItemName
                                : string.Empty
                    Item2Price = secondItemExists
                                ? secondItem.ItemPrice
                                : null
                    Item3Name = thirdItemExists
                                ? thirdItem.ItemName
                                : string.Empty
                    Item3Price = thirdItemExists
                                ? thirdItem.ItemPrice
                                : null
                });

,

此查询应能满足您的要求，但是我没有测试LINQ to SQL是否可以翻译它：

var ans = (from o in Orders
           join oi in OrderItems on o.OrderId equals oi.OrderId
           join i1 in Items on oi.ItemId1 equals i1.ItemId into i1j
           from i1 in i1j.DefaultIfEmpty()
           join i2 in Items on oi.ItemId2 equals i2.ItemId into i2j
           from i2 in i2j.DefaultIfEmpty()
           join i3 in Items on oi.ItemId3 equals i3.ItemId into i3j
           from i3 in i3j.DefaultIfEmpty()
           select new {
               o.OrderDate,Item1Name = i1.ItemName,Item1Price = oi.Price1,Item2Name = i2.ItemName,Item2Price = oi.Price2,Item3Name = i3.ItemName,Item3Price = oi.Price3,})
          .ToList();