使用 Linq 从 Northwind DB 生成报告答案

【问题标题】：Generate a report from Northwind DB using Linq使用 Linq 从 Northwind DB 生成报告
【发布时间】：2013-03-11 16:58:53
【问题描述】：

我正在尝试使用 Linq 从流行的 NorthWind DB 生成以下报告。它应该由Customer、OrderYear分组。

CustomerName    OrderYear    Amount

我必须使用下表Customer、Order 和Order Details。到目前为止，这就是我所做的。

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();
        }

它给了我预期的结果。我通过在后端运行 t-sql 进行比较。但是，我有 2 个问题。

在内部foreach 中，第二条语句生成总和。这是正确的方法吗？还是有更好的？
如何在 Linq 查询本身中获取 Sum。

【问题讨论】：

标签： c# .net linq linq-to-sql northwind

【解决方案1】：

在单个 LINQ to SQL 查询中得到它：

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)
            };

然后您可以简单地获得结果：

var results = query.ToList();

或者在获取之前对其进行排序：

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

我对 LINQ to SQL 查询生成的 SQL 很好奇，所以设置自定义 Log 如下：

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

有点吓人，不是吗？但是如果你仔细观察，有一个标准的LEFT JOIN 用于将所有三个表组合在一起！其余的只是分组、排序和求和。

【讨论】：