[英]Applying orderby to C# LINQ Inner join
我正在从这个msdn页面尝试C#LINQ Joins。
即使它适用于群组加入,我也无法通过内部联接应用订单。
运行查询的数据是:
class Product
{
public string Name { get; set; }
public int CategoryID { get; set; }
}
class Category
{
public string Name { get; set; }
public int ID { get; set; }
}
// Specify the first data source.
static List<Category> categories = new List<Category>()
{
new Category(){Name="Beverages", ID=001},
new Category(){ Name="Condiments", ID=002},
new Category(){ Name="Vegetables", ID=003},
new Category() { Name="Grains", ID=004},
new Category() { Name="Fruit", ID=005}
};
// Specify the second data source.
static List<Product> products = new List<Product>()
{
new Product{Name="Cola", CategoryID=001},
new Product{Name="Tea", CategoryID=001},
new Product{Name="Mustard", CategoryID=002},
new Product{Name="Pickles", CategoryID=002},
new Product{Name="Carrots", CategoryID=003},
new Product{Name="Bok Choy", CategoryID=003},
new Product{Name="Peaches", CategoryID=005},
new Product{Name="Melons", CategoryID=005},
};
所需的输出是 (按括号中的类别列出顺序,然后按产品排序):
Cola(Beverages)
Tea(Beverages)
Mustard(Condiments)
Pickles(Condiments)
Melons(Fruit)
Peaches(Fruit)
Bok Choy(Vegetables)
Carrots(Vegetables)
我能够使用orderby
group-join
和orderby
以及第二个from-select
来生成此输出,以使组层次结构变形并生成如下的简单列表:
var listGroupJoinOrderBy =
from category in categories
join product in products on category.ID equals product.CategoryID into prodGroup
from prod in prodGroup
orderby category.Name, prod.Name //first order by category name then product name
select
new
{
Category = category.Name,
Product = prod.Name
};
但是后来我无法使用orderby
应用于内连接(即没有into
子句的group-join)生成相同的输出。 我试过以下变种:
变体#1
var innerJoinOrderBy =
from category in categories
orderby category.Name //orderby category name
join product in products on category.ID equals product.CategoryID
orderby product.Name //orderby product name
select
new
{
Category = category.Name,
Product = product.Name
};
变种#2
var innerJoinOrderBy =
from category in categories
join product in products on category.ID equals product.CategoryID
orderby category.Name, product.Name //orderby category first and then by product name
select
new
{
Category = category.Name,
Product = product.Name
};
但是,两个变体都提供相同的输出,就像没有使用orderby
,并产生以下输出:
Cola (Beverages)
Tea (Beverages)
Mustard (Condiments)
Pickles (Condiments)
Carrots (Vegetables)
Bok Choy (Vegetables)
Peaches (Fruit)
Melons (Fruit)
问:如何使用inner-join
和orderby
生成所需的输出?
无论如何,要打印查询结果,可以使用以下foreach
(只需更改查询变量名称):
foreach (var product in simpleInnerJoin)
{
Console.WriteLine(product.Product + " (" + product.Category + ")");
}
你的第二个选择应该工作正常
var innerJoinOrderBy =
from c in categories
join p in products on c.ID equals p.CategoryID
orderby c.Name, p.Name
select new {
Category = c.Name,
Product = p.Name
};
输出:
[
{ Product="Cola", Category="Beverages" },
{ Product="Tea", Category="Beverages" },
{ Product="Mustard", Category="Condiments" },
{ Product="Pickles", Category="Condiments" },
{ Product="Melons", Category="Fruit" },
{ Product="Peaches", Category="Fruit" },
{ Product="Bok Choy", Category="Vegetables" },
{ Product="Carrots", Category="Vegetables" }
]
从您的输出中我看到项目按原始顺序排列。 确保在查询中应用了orderby
运算符。
将Orderby从主Linq中取出并使用
foreach (var product in simpleInnerJoin.Orderby(i=> i.Category.Name).ThenBy(i=>i.Product.Name))
{
Console.WriteLine(product.Product + " (" + product.Category + ")");
}
我认为这应该为您提供所需的输出。
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