将简单的SQL组by转换为LINQ to SQL

本文关键字：SQL LINQ to 转换 简单 by | 更新日期: 2023-09-27 18:01:20

我遇到麻烦了。我无法理解Stack Overflow上已有的答案，而且我对LINQ to SQL太陌生了，无法自己解决这个问题。

请参阅此SQL:

select p.Name as ProductName, SUM(o.NumberOf) as TotalOrdered from [Order] o
  join [Product] p on o.ProductId = p.Id
  group by p.Name

返回一个漂亮的两列表，左边是产品名称，右边是已订购的产品总数(所有订单(。如何在LINQ to SQL中复制此项？

到目前为止，我得到的是：

var ctx = new DataClasses1DataContext();
var totalProducts = (from o in ctx.Orders
                     join p in ctx.Products on o.ProductId equals p.Id
                     select new { p.Name, o.NumberOf })
    .GroupBy(t => t.Name)
    .Select(g => g.Key, ... );

什么在？

将简单的SQL组by转换为LINQ to SQL

在我看来，它就像你想要的：

.Select(g => new { ProductName = g.Key, TotalOrdered = g.Sum(x => x.NumberOf) })

您可以将整个查询作为单个查询表达式进行，也可以完全不使用查询表达式：

var totalProducts = ctx.Orders
                       .Join(ctx.Products, o => o.ProductId, p => p.Id,
                             (o, p) => new { p.Name, o.NumberOf })
                       .GroupBy(t => t.Name,
                                pair => pair.Name, // Key selector
                                pair => pair.NumberOf, // Element selector
                                (key, numbers) => new { 
                                    ProductName = key,
                                    TotalOrdered = numbers.Sum()) 
                                });

或者：

var totalProdcuts = from o in ctx.Orders
                    join p in ctx.Products on o.ProductId equals p.Id
                    group o.NumberOf by p.Name into g
                    select new { ProductName = g.Key, TotalOrdered = g.Sum() };

TotalOrdered = g.Sum(o => o.NumberOf)

利用上面的。。。。则语句可能是

   select new { ProductName=  g.Key, TotalOrdered  = g.Sum(o => o.NumberOf) };
var query = from o in ctx.Orders
            join p in ctx.Products on 
            o.ProductId equals p.Id
            group o by new { p.Name, o.NumberOf } into g
            select new { ProductName=  g.Key, TotalOrdered  = g.Sum(o => o.NumberOf) };

只需将其粘贴在此处，以防了解如何通过GroupJoin方法实现这一点很有帮助：

var totalProducts = ctx.Products.GroupJoin(ctx.Orders,
    outerKeySelProduct => outerKeySelProduct.Id,
    innerKeySelOrder => innerKeySelOrder.ProductId,
    (outerKeySelProduct, innerKeySelOrder) => new
    {
        ProductName = outerKeySelProduct.Name,
        TotalOrdered = innerKeySelOrder.Select(n => n.NumberOf).Sum()
    });

如果可以改进的话，很高兴能对它进行编辑。

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