如何对 linq 查询进行分组和子分组以及获取数据答案

【问题标题】：How to group and subgroup and get data from a linq query如何对 linq 查询进行分组和子分组以及获取数据
【发布时间】：2019-04-14 16:20:18
【问题描述】：

嗨。我有三个表，我正在尝试将它们分组和子组。

Table Sections
-------------
Id
SectionName


Table Categories
-------------
Id 
CategoryName
SectionRefId


Table Records
-------------
Id
RecordName
CategoryRefId

我想要实现的是按 SectionName 对所有类别进行分组，并按 CategoryName 对所有记录进行分组，并使用 foreach 循环显示它们。

I tried this using Linkpad the result it not what i exptected </p>


var result = from doc in Categories
             group doc by doc.SectionRefId into docSections
             select new  
             {
                Name = docSections.Key,
                Group = from dl in Records
                        group dl by dl.CategoryRefId into dlRecords
                        select new 
                        {
                            Name = dlRecords.Key,
                            GroupRecords = dlocation
                        }
             };


enter code here

【问题讨论】：

你有相同的sql查询吗？
无 sql 查询，我试图仅使用 linq 来解决。尝试以下解决方案。

标签： c# entity-framework linq model-view-controller

【解决方案1】：

你可以这样做。需要一些加入和分组：

var result = from category in Categories
             join section in Sections on category.Id equals section.ID
             join record in Records on category.Id equals record.CategoryRefId
             group category by {section.SectionName} into g
             group record by {category.categoryName} into p
             select new { CategoryName = g.Key.CategoryName, SectionName = section.SectionName, RecordName = record.RecordName };

【讨论】：

【解决方案2】：

如果您遵循entity framework code first conventions，您的类将具有virtual ICollection<...> 属性，可以为您进行分组：

class Section
{
    public int Id { get; set; }
    public string SectionName { get; set; }

    // every section has zero or more Categories (one-to-many)
    public virtual ICollection<Category> Categories {get; set;}
}

class Category
{
    public int Id { get; set; }
    public string CategoryName { get; set; }

    // every Category belongs to exactly one Section using foreign key:
    public int SectionId { get; set; }
    public virtual Section Section {get; set;}

    // every Category has zero or more Records (one-to-many)
    public virtual ICollection<Record> Records {get; set;}
}

class Record
{
    public int Id { get; set; }
    public string RecordName { get; set; }

    // every Record belongs to exactly one Category
    public int CategoryId { get; set; }
    public virtual Category Category {get; set;}
}

在实体框架中，表示数据库表的列由非虚属性。虚拟属性代表表之间的关系

注意，你的表和列可能有不同的标识符，主要是你添加了虚拟属性

我想要实现的是按 SectionName 对所有 Category 进行分组，并按 CategoryName 对所有 Records 进行分组，并使用 foreach 循环显示它们。

var results = myDbContext.Sections
    .Where (section => ...)             // only if you don't want all Sections
    .Select(section => new
    {
        // select only the properties you plan to use:
        Id = section.Id,
        Name = section.SectionName,

        // This section has zero or more categories:
        Categories = section.Categories
           .Where(category => ...)          // only if you don't want all categories
           .Select(category => new
           {
               // again, select only the properties you plan to use:
               Id = category.Id,
               ...
               // not needed, you already know the value:
               // SectionId = category.SectionId,

               // this category has zero or more Records:
               // you know the drill by now
               Records = category.Records
                  .Where(record => ...)   
                  .Select(record => new
                  {
                      Id = record.Id,
                      ...
                  })
                  .ToList(),
           })
           .ToList(),
     });

实体框架知道您的一对多关系，并会为您执行适当的 GroupJoins

【讨论】：