Question

嗨我有以下代码来Customers Advertureworks2008R2LT和SalesPerson对客户＆＃39;进行分组last name。 var dc = new DataClasses1DataContext(); var groupedCust = (from c in dc.Customers group c by new { c.SalesPerson, c.LastName }).Take(10); foreach (var item in groupedCust) { Console.WriteLine("\n***{0}***", item.Key.SalesPerson); Console.WriteLine("=========================="); Console.WriteLine("\n{0}", item.Key.LastName); Console.WriteLine("-------------------------"); foreach (var item2 in item) { Console.WriteLine("{0}, {1}", item2.LastName, item2.FirstName); } }。

LINQ2SQL

我正在使用"adventure-works\shu0"仅用于演示。问题是它没有与名为adventure-works\shu0的销售人员进行分组，在"adventure-works\linda3"之间存在名为linda3的销售人员。但这不应该发生，因为shu0必须在所有***adventure-works\pamela0*** ========================== Gee ------------------------- Gee, Orlando Gee, Orlando ***adventure-works\david8*** ========================== Harris ------------------------- Harris, Keith Harris, Keith ***adventure-works\jillian0*** ========================== Carreras ------------------------- Carreras, Donna Carreras, Donna ***adventure-works\jillian0*** ========================== Gates ------------------------- Gates, Janet Gates, Janet ***adventure-works\shu0*** ========================== Harrington ------------------------- Harrington, Lucy Harrington, Lucy ***adventure-works\linda3*** ========================== Carroll ------------------------- Carroll, Rosmarie Carroll, Rosmarie ***adventure-works\shu0*** ========================== Gash ------------------------- Gash, Dominic Gash, Dominic ***adventure-works\josé1*** ========================== Garza ------------------------- Garza, Kathleen Garza, Kathleen ***adventure-works\josé1*** ========================== Harding ------------------------- Harding, Katherine Harding, Katherine ***adventure-works\garrett1*** ========================== Caprio ------------------------- Caprio, Johnny Caprio, Johnny之后。为什么会发生这种情况，我该如何解决这个问题？

这是查询执行的结果：

num_vertices, num_edges = gets.chomp.split(' ').map { |e| e.to_i }
graph = Graph.new
(1..num_vertices).to_a.each do |vertex|
  graph.add_node_by_val(vertex)
end

num_edges.times do |edge|
  first, second = gets.chomp.split(' ').map { |e| e.to_i }
  graph.add_edge_by_val(first, second, 0, false)
end

even_edges = 0
graph.edges.each do |edge|
  dup = graph.deep_dup
  first_tree = nil
  second_tree = nil
  subject_edge = nil
  dup.edges.each do |e|
    if e.first.value == edge.first.value && e.second.value == edge.second.value
      subject_edge = e
      first_tree = e.first
      second_tree = e.second
    end
  end
  dup.remove_edge(subject_edge)
  if first_tree.size.even? && second_tree.size.even?
    even_edges += 1
  end
end
puts even_edges

Answer 1

这里的问题是，与LINQ to Objects的处理方式相比，LINQ to SQL / LINQ to Entities如何处理分组之间存在差异。

基本上，当您在L2S / L2E中使用groupby子句（或GroupBy方法）时，将从生成的SQL中删除排序。结果是SQL服务器将以您无法控制的任意顺序返回结果...除非您在组操作后指定排序。

var groupedCust =
(
    from c in dc.Customers
    group c by new { c.SalesPerson, c.LastName } into grp
    orderby grp.Key.SalesPerson, grp.Key.LastName
    select grp
).Take(10);

这应该产生你所追求的输出，但是在LINQ to SQL中查询组时会有一个警告。当您随后枚举结果时，LINQ将为具有组键特征的每个组发出查询，以获取与该组对应的记录。这意味着多次往返数据库，而不是仅仅开始一堆记录。

要从数据库中获取所需的记录（只有您想要的记录），您可以加入组查询，获取匹配的记录，然后将它们重新组合到内存中。

或者把它放在代码中：

// when enumerated this will result in one SQL statement
var groupingSource = 
(
    from key in
    (
        from gc in dc.Customers
        group 1 by new { gc.SalesPerson, gc.LastName } into grp
        orderby grp.Key.SalesPerson, grp.Key.LastName
        select grp.Key
    ).Take(10)

    join c in dc.Customers on key equals new { c.SalesPerson, c.LastName }
    select c
);

// get the records and re-group them
var groupedCust = 
    from c in groupingSource.AsEnumerable()
    group c by new { c.SalesPerson, c.LastName } into grp
    orderby grp.Key.SalesPerson, grp.Key.LastName
    select grp;

其中的AsEnumerable会将记录从数据库中拉入内存，并将表达式的其余部分作为LINQ to Objects ...来完成，只需一次调用数据库即可获得预期的结果而不是11个单独的电话。当你从数据库中删除 lot 组时它会产生真正的不同，请相信我。

Answer 2

您获得的输出对于您已经给出的LINQ / SQL来说非常好。

但试试这个，看看你是否得到了你想要的结果：

var groupedCust =
(
    from c in dc.Customers
    orderby c.LastName
    orderby c.SalesPerson
    group c by new { c.SalesPerson, c.LastName }
).Take(10);

为什么分组不能正确分组salesPerson和LastName？

2 个答案: