左连接Linq并使用变量为其他连接

Question

当我使用Linq中的左连接为DataSet时，我卡住的错误“值不能为空参数名称行”。

以下是数据和linq查询。

• DataRowCollection - 项目（列 - ITEM_ID，SKU，数量）
• DataRowCollection - 优惠（色谱柱 - ITEM_ID，Promotion_Id）
• DataRowCollection - 组分（色谱柱 - COMPONENT_ID，Promotion_id）
• DataRowCollection - 金额（列 - Component_id，Amount_Text，货币）

 var q = 
      from Item in items 
      join promotion in promotions 
       on Item.Field<int>("Item_id") equals promotion.Field<int?>("Item_id") into promo 
      from promotion in promo.DefaultIfEmpty() 
      join disccomponent in components 
       on promotion.Field<int>("Promotion_Id") equals disccomponent.Field<int?>("Promotion_Id") 
      join discamounts in amounts 
       on disccomponent.Field<int>("Component_id") equals discamounts.Field<int?>("Component_id") 
      where disccomponent.Field<string>("Type") == "Principal" 
      select new 
      { 
       SKU = Item.Field<string>("SKU"), 
       Quantity = Item.Field<string>("Quantity"), 
       DiscountAmount = discamounts.Field<string>("Amount_Text"), 
       DiscountCurrency = discamounts.Field<string>("currency") 
      }; 

我需要得到以下几点：

• SKU，数量和折扣的细节对于具有打折商品
• SKU，数量为不具有任何折扣

代码工作没有项目当所有物品都有折扣时左外连接，但如果任何物品没有任何折扣，则跳过该物品，因此我不得不使用左外连接。

任何帮助，高度赞赏。请让我知道是否需要澄清。

在此先感谢。

Answer 1

谢谢大家对你的帮助。我通过将它分解成下面的2个不同的linq查询来完成这项工作。

第一个查询得到推广数据的VAR

 var promoData = 
       from promotion in promotions 
       join component in components 
        on promotion.Field<int>("Promotion_Id") equals component.Field<int?>("Promotion_Id") 
       join amount in amounts 
        on component.Field<int>("Component_id") equals amount.Field<int?>("Component_id") 
       where component.Field<string>("Type") == "Principal" 
       select new 
       { 
        Item_id = promotion.Field<int?>("Item_id"), 
        Promotion_id = promotion.Field<int>("Promotion_Id"), 
        DiscountAmount = amount == null ? "" : amount.Field<string>("Amount_Text"), 
        DiscountCurrency = amount == null ? "" : amount.Field<string>("currency") 
       }; 

第二个查询使用促销VAR简化查询并获得resuls

 var q = 
      from Item in items 
      join promotion in promoData 
       on Item.Field<int>("Item_id") equals promotion.Item_id into promo 
       from promoJoin in promo.DefaultIfEmpty() 
      select new 
      { 
       SKU = Item.Field<string>("SKU"), 
       Quantity = Item.Field<string>("Quantity"), 
       DiscountAmount = promoJoin != null ? promoJoin.DiscountAmount : "0", 
       DiscountCurrency = promoJoin != null ? promoJoin.DiscountCurrency : "" 
      }; 

这种方法适用完美地获得所有物品，无论他们是否有促销活动。我仍然在考虑是否可以在单个查询中做同样的事情:) 现在将其标记为答案，如果有人想出其他更好的方法，那么标记就是这样。

Answer 2

我认为你试图访问属性为null discamounts。也许你的选择应新这个样子......

select new 
{ 
    SKU = Item.Field<string>("SKU"), 
    Quantity = Item.Field<string>("Quantity"), 
    DiscountAmount = discamounts == null ? "" : discamounts.Field<string>("Amount_Text"), 
    DiscountCurrency = discamounts == null ? "" : discamounts.Field<string>("currency") 
}; 

Answer 3

给你的使用into加入一个名称，就可以使用DefaultIfEmpty。像这样的事情（看在声明的最后join和from表达）

var q = 
    from Item in items 
    join promotion in promotions 
     on Item.Field<int>("Item_id") equals promotion.Field<int?>("Item_id") into promo 
    from promotion in promo.DefaultIfEmpty() 
    join disccomponent in components 
     on promotion.Field<int>("Promotion_Id") equals disccomponent.Field<int?>("Promotion_Id") 
    join discamounts in amounts 
     on disccomponent.Field<int>("Component_id") equals discamounts.Field<int?>("Component_id") into DiscamountJoin 
    from discamount in DiscamountJoin.DefaultIfEmpty() 
    where disccomponent.Field<string>("Type") == "Principal" 
    select new 
    { 
     SKU = Item.Field<string>("SKU"), 
     Quantity = Item.Field<string>("Quantity"), 
     DiscountAmount = discamount.Field<string>("Amount_Text"), 
     DiscountCurrency = discamount.Field<string>("currency") 
    };