C# 按品牌然后按类别对产品进行分组-linq

有三个表：

Brand                               Category
-----------------------          --------------------------------
BrandID     BrandName             CategoryID        CategoryName
-----------------------          --------------------------------
1           Brand1                    1             cat1
2           Brand2                    2             cat2

Product
------------------------------------------------------------
ProductID       BrandID      CategoryID    ProductName
------------------------------------------------------------   
1                 1             1          Product1
2                 1             1          Product2
3                 1             2          Product3
4                 2             1          Product4

我想编写Llinq查询以获得如下输出：

Brand1
|__ cat1
|   |__ Product1
|   |__ Product2
|
|__ Cat2
|   |__ Product3
|
|__ Brand2
|__ cat1
|   |__ Product4

我编写此查询是为了按类别获取产品组：

    var query2 = db.Brands
        .GroupBy(x => new { x.BrandID, x.BrandName })
                    .Select(x => new
                    {
                        x.Key.BrandID,
                        x.Key.BrandName,
                        ProductsNames = db.Products
                                            .Where(p => p.BrandID == x.Key.BrandID)
                                            .Select(p => p.ProductName).ToList()
                    }).ToList();

---

如何按品牌然后按类别对产品进行分组？

您可以通过两个步骤完成此操作

//Step 1 - Join 3 lists
var query = (from p in products
                join b in brands on p.BrandId equals b.Id
                join c in categories on p.CategoryId equals c.Id
                select new
                {
                    b.BrandName,
                    c.CategoryName,
                    p.ProductName
                }).ToList();

//Step 2 - query required results
var results = query
                .GroupBy(x => new { x.BrandName, x.CategoryName })
                .Select(x => new
                {
                    Brand = x.Key.BrandName,
                    Category = x.Key.CategoryName,
                    Product = x.Select(y=>y.ProductName).ToList()
                });

Console.WriteLine(JsonConvert.SerializeObject(results));

输出

[
    {
        "Brand": "Brand1",
        "Category": "cat1",
        "Product": [
            "Product1",
            "Product2"
        ]
    },
    {
        "Brand": "Brand1",
        "Category": "cat2",
        "Product": [
            "Product3"
        ]
    },
    {
        "Brand": "Brand2",
        "Category": "cat1",
        "Product": [
            "Product4"
        ]
    }
]

---

我写了一个小控制台程序：

class Program
{
    static void Main(string[] args)
    {
        var products = new List<Product>{
            new Product { ProductID = 1, BrandID = 1, CategoryID = 1, ProductName = "Product1" },
            new Product { ProductID = 2, BrandID = 1, CategoryID = 1, ProductName = "Product2" },
            new Product { ProductID = 3, BrandID = 1, CategoryID = 2, ProductName = "Product3" },
            new Product { ProductID = 4, BrandID = 2, CategoryID = 1, ProductName = "Product4" }
        };
        var groupedByBrand = 
            products
                .GroupBy(p => p.BrandID)
                .Select(g => g.GroupBy(p => p.CategoryID));
        foreach (var groupByBrand in groupedByBrand)
            foreach (var groupByCategory in groupByBrand)
                foreach (var product in groupByCategory)
                    Console.WriteLine($"{product.BrandID} - {product.ProductName}");
    }
}

public class Product
{
    public int ProductID { get; internal set; }
    public int BrandID { get; internal set; }
    public int CategoryID { get; internal set; }
    public string ProductName { get; internal set; }
}

我按BrandId分组，然后在这个组中按categoryId分组

如果只是关于分组，您可以使用BrandID和CategoryID，甚至不需要加入表。如果您想按名称排序，那么当然需要加入

---

也许你想在分组的基础上做一些排序，而不是分组？

一步到位，更加紧凑

public class Product
    {
        public int ProductID { get; internal set; }
        public string ProductName { get; internal set; }
        public Brand brand { get; set; }
        public Category cat { get; set; }
        public Product()
        {
            brand = new Brand();
            cat = new Category();
        }
    }

    public class Brand
    {
        public int BrandID { get; internal set; }
        public string BrandName { get; internal set; }
    }

    public class Category
    {
        public int CategoryID { get; internal set; }
        public string CategoryName { get; internal set; }
    }

private void a1()
{
    var products = new List<Product>{
    new Product { ProductID = 1,
                                brand =new Brand { BrandID = 1, BrandName = "Brand1" },
                                cat = new Category { CategoryID = 1, CategoryName = "cat1" },
                                ProductName = "Product1" },
    new Product { ProductID = 2,
                                brand =new Brand { BrandID = 1, BrandName = "Brand1" },
                                cat = new Category { CategoryID = 1, CategoryName = "cat1" },
                                ProductName = "Product2" },
    new Product { ProductID = 3,
                                brand =new Brand { BrandID = 1, BrandName = "Brand1" },
                                cat = new Category { CategoryID = 2, CategoryName = "cat2" },
                                ProductName = "Product3" },
    new Product { ProductID = 4,
                                brand =new Brand { BrandID = 2, BrandName = "Brand2" },
                                cat = new Category { CategoryID = 1, CategoryName = "cat1" },
                                ProductName = "Product4" }
    };

    var g1 = products.GroupBy(p => new { p.brand, p.cat, p })
                        .Select(y => new 
                        {
                            k1 = y.Key.brand.BrandName,
                            k2 = y.Key.cat.CategoryName,
                            k3 = y.Key.p.ProductName
                        }       
                        );
}

// Result/Output as below
{ k1 = "Brand1", k2 = "cat1", k3 = "Product1" }
{ k1 = "Brand1", k2 = "cat1", k3 = "Product2" }
{ k1 = "Brand1", k2 = "cat2", k3 = "Product3" }
{ k1 = "Brand2", k2 = "cat1", k3 = "Product4" }

---

您使用的是没有类别的db.Brands表。而是使用表db.Products.Yes，我需要向用户显示品牌和类别的名称。