c# - 跨多个集合的 linq 查询总和产品

Question

我有两个如下所示的数据集：

+------------------------------------+
| Products                           |
+------------------------------------+
| Id | Name             | Price      |
+------------------------------------+
|  1 | apples           | 1.00       |
|  2 | oranges          | 2.00       |
|  3 | pomengrate       | 3.00       |
+------------------------------------+

+-------------------------------+
| Sales                         |
+-------------------------------+
| CustId | ProductId | Quantity |
+-------------------------------+
|      1 |         1 |        5 |
|      1 |         2 |        4 |
|      1 |         3 |        2 |
|      2 |         1 |        8 |
|      2 |         3 |        7 |
+-------------------------------+     

我想得到每个客户花费的金额，最终得到如下结果：

+----------------+
| CustId | Total |
+----------------+
|      1 | 19.00 |
|      2 | 29.00 |
+----------------+

我可以对单个表进行求和，但 LINQ 中的 Sum 方法采用只有一个参数的 lambda：对要求和的值所在的表的引用。这些值位于不同的表中。我如何将这些加在一起？

Answer 1

var totals =
    from sale in Sales
    group sale by sale.CustId into custSales
    select new
    {
        CustId = custSales.Key, 
        Total = (
            from custSale in custSales
            select custSale.Product.Price *
                custSale.Quantity)
            .Sum()
    };

Answer 2

试一试。它给出了您正在寻找的结果：

var results = sales.Join(products,
    sale => sale.ProductID,
    product => product.ID,
    (sale, product) => new { CustID = sale.CustID, Total = sale.Quantity * product.Price })
    .GroupBy(r => r.CustID)
    .Select(g => new { CustID = g.Key, Total = g.Sum(gt => gt.Total) });

Answer 3

为了完整起见，这里是查询语法版本（使用连接而不是子选择）：

var totals =
    from sale in sales
    join prod in product on sale.ProductId equals prod.Id
    let saleProds = new { sale.CustId, Total = prod.Price * sale.Quantity }
    group saleProds by saleProds.CustId into custSale
    select new { Customer = custSale.Key, Total = custSale.Sum(tr => tr.Total) };

关键部分是您需要以某种方式将连接的集合结果（销售和产品）转换为单个实体，然后可以对其进行分组。