使用Linq從Northwind數據庫生成報告

Question

我正在嘗試使用Linq從流行的NorthWind DB生成以下報告。 它應該按Customer OrderYear 。

CustomerName    OrderYear    Amount

我必須使用以下表格Customer ， Order和Order Details 。 到目前為止，這是我所做的。

NorthwindDataContext north = new NorthwindDataContext();

        var query = from o in north.Orders
                    group o by o.Customer.CompanyName  into cg
                    select new
                    {
                        Company = cg.Key,
                        YearGroup = (   from y in cg
                                              group y by y.OrderDate.Value.Year into yg
                                              select new 
                                              {
                                                  Year = yg.Key,
                                                  YearOrdes = yg
                                              }                                             
                                         )
                    };


        foreach (var q in query)
        {
            Console.WriteLine("Customer Name : " + q.Company);
            foreach (var o in q.YearGroup)
            {
                Console.WriteLine("Year " + o.Year);
                Console.WriteLine("Sum " + o.YearOrdes.Sum(yo => yo.Order_Details.Sum( yd=> Convert.ToDecimal(yd.UnitPrice* yd.Quantity))));           
            }

            Console.WriteLine();
        }

它給了我預期的結果。 我通過在后端運行t-sql進行比較。但是，我有兩個問題。

1. 在Inner foreach ，第二條語句生成和。 這是適當的方法嗎？ 還是有更好的選擇？
2. 如何在Linq查詢本身中獲得Sum。

Answer 1

在單個LINQ to SQL查詢中得到它：

var query = from o in north.Orders
            from c in north.Customers.Where(c => c.CustomerID == o.CustomerID).DefaultIfEmpty()
            from d in north.Order_Details.Where(d => d.OrderID == o.OrderID).DefaultIfEmpty()
            group new { o, c, d } by new { o.OrderDate.Value.Year, c.CompanyName } into g
            select new
            {
                Company = g.Key.CompanyName,
                OrderYear = g.Key.Year,
                Amount = g.Sum(e => e.d.UnitPrice * e.d.Quantity)
            };

然后，您可以簡單地得到結果：

var results = query.ToList();

或在獲取之前對其進行排序：

var results = query.OrderBy(g => g.Company).ThenByDescending(g => g.OrderYear).ToList();

我對由LINQ to SQL查詢生成的SQL感到好奇，因此設置自定義Log ，如下所示：

SELECT [t5].[value22] AS [Company], [t5].[value2] AS [OrderYear], [t5].[value] AS [Amount]
FROM (
    SELECT SUM([t4].[value]) AS [value], [t4].[value2], [t4].[value22]
    FROM (
        SELECT [t3].[UnitPrice] * (CONVERT(Decimal(29,4),[t3].[Quantity])) AS [value], [t3].[value] AS [value2], [t3].[value2] AS [value22]
        FROM (
            SELECT DATEPART(Year, [t0].[OrderDate]) AS [value], [t1].[CompanyName] AS [value2], [t2].[UnitPrice], [t2].[Quantity]
            FROM [dbo].[Orders] AS [t0]
            LEFT OUTER JOIN [dbo].[Customers] AS [t1] ON [t1].[CustomerID] = [t0].[CustomerID]
            LEFT OUTER JOIN [dbo].[Order Details] AS [t2] ON [t2].[OrderID] = [t0].[OrderID]
            ) AS [t3]
        ) AS [t4]
    GROUP BY [t4].[value2], [t4].[value22]
    ) AS [t5]
ORDER BY [t5].[value22], [t5].[value2] DESC
-- Context: SqlProvider(Sql2008) Model: AttributedMetaModel Build: 3.5.30729.6387

有點嚇人，不是嗎？ 但是，如果您仔細觀察，可以使用標准的LEFT JOIN將所有三個表組合在一起！ 其余的只是分組，排序和求和。