Question

我正在尝试this msdn page的C＃LINQ联接。

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（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group-join orderby和第二from-select来生成此输出，以使组层次结构变形并生成普通列表，如下所示：

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
            };

但是后来我无法生成同样的输出，orderby应用于内连接（即没有into子句的group-join）。我尝试了以下变体：

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
            };

变体＃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一样，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 + ")");
}

Answer 1

您的第二个选项应该可以正常使用

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其原始顺序排列。确保您已在查询中应用orderby运算符。

Answer 2

从主Linq中取出Orderby并使用

foreach (var product in simpleInnerJoin.Orderby(i=> i.Category.Name).ThenBy(i=>i.Product.Name))
{
    Console.WriteLine(product.Product + " (" + product.Category + ")");
}

我认为这应该可以为您提供所需的输出。

将orderby应用于C＃LINQ内连接

2 个答案: