通过关联类检索数据答案

【问题标题】：Retrieving data through associative class通过关联类检索数据
【发布时间】：2018-11-06 19:54:52
【问题描述】：

我有两个表，Animal 和 Circus，它们具有多对多关系，产生了关联类 Contract，我有一个 Breed 类，通过一对多关系与 Animal 相关联。我怎样才能找到马戏团中动物数量最多的品种。

我试过了：

var req = from Animal A in Contract
                     group A by A.Breed into g
                     select max { g.Key, Count = g.Count() };

但它不起作用我的意思是，如果我们有：

**Animal_FK**      **Circus_FK**     **Salary**    **Contract_ date**      
----------          ----------      ------------    -------------------

   S3                  C10             3000           2015-12-12

   S1                  C12             5000           2010-10-17

   D2                  C18             4000           2016-02-22

   B4                  C85             3500           2018-09-30

   S2                   C6              2700           2014-05-10


Animal_key        Name          
-------------  -------------          
    S1            Python 
    S2            Cobra              
    S3            Boa              
    D1            Bulldog              
    D2            Labrador              
    B1            Owl
    B2            Dove   





Beer_Key      Name         Description    
----------  ---------    ----------------                    
   S          Snake            reptile      
   D          Dog              pet     
   B          Birds            flying animals

查询结果将是Snake

【问题讨论】：

为什么是标签？
@jarlh 因为是sql请求
那不是 SQL。也许是 linq？
您是指在马戏团中出场的动物最多的品种（有一份或多份合同），还是出场次数最多的品种（合同总数最高）？我想我们需要一些样本数据和您的预期结果。
合约总数最高的品种

标签： c# asp.net .net entity-framework linq

【解决方案1】：

我假设这里有一些 POCO。我不会提供薪水等其他详细信息，只提供最低限度。

public class Breed
{
    public string Name { get; set; }
}

public class Animal
{
    public string Name { get; set; }
    public Breed Breed { get; set; }
}

public class Circus
{
    public string Name { get; set; }
}

public class Contract
{
    public Animal Performer { get; set; }
    public Circus Venue { get; set; }
}

然后设置一些代表你上面的数据。

var breeds = new[]
{
    new Breed {Name = "Snake"},
    new Breed {Name = "Dog"},
    new Breed {Name = "Bird"}
};

var animals = new[]
{
    new Animal{Name="Boa", Breed = breeds[0]},     // 0
    new Animal{Name="Python", Breed = breeds[0]},  // 1
    new Animal{Name="Cobra", Breed = breeds[0]},   // 2
    new Animal{Name="Bulldog", Breed = breeds[1]}, // 3
    new Animal{Name="Labrador", Breed = breeds[1]},// 4
    new Animal{Name="Dove", Breed = breeds[1]},    // 5
    new Animal{Name="Owl", Breed = breeds[2]},     // 6
};

var circuses = new[]
{
    new Circus {Name = "C1"},
    new Circus {Name = "C2"},
    new Circus {Name = "C3"},
    new Circus {Name = "C4"},
    new Circus {Name = "C5"},
};

var contracts = new[]
{
    new Contract {Performer = animals[0], Venue = circuses[0]},
    new Contract {Performer = animals[1], Venue = circuses[1]},
    new Contract {Performer = animals[2], Venue = circuses[2]},
    new Contract {Performer = animals[4], Venue = circuses[3]},
    new Contract {Performer = animals[6], Venue = circuses[4]},
};

那么这就是你获得最具代表性的品种的方法：

var mostRepresentedBreed = (
        from contract in contracts
        group contract by contract.Animal.Breed
        into contractsByBreed
        orderby contractsByBreed.Count() descending
        select contractsByBreed.Key
    ).FirstOrDefault();

var theWinnerIs = mostRepresentedBreed.Name;

希望这会有所帮助。布雷特。

【讨论】：