C# 如何按某个属性对一个表进行分组，然后对LINQ中其他表（带有外键）中的元素进行计数？

假设我有两个表（实体）：

我想按“城市”属性对所有人员实体进行分组，并计算每个城市在JoinTable中引用的人员数量。

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我如何在LINQ中查询这个问题？

我不太确定，您想获得什么。但我认为是这样的：

// Example Data (would be great if you could write some in your questions..)
List<Person> ps = new List<Person>()
{
    new Person() { Id = 1, City = "Cologne" },
    new Person() { Id = 2, City = "Cologne" },
    new Person() { Id = 3, City = "Berlin" },
    new Person() { Id = 4, City = "Berlin" },
};
List<JoinTable> join = new List<JoinTable>()
{
    new JoinTable() { Id = 1, Person_Id = 1, SomeOther_Id = 1000 },
    new JoinTable() { Id = 1, Person_Id = 1, SomeOther_Id = 2000 },
    new JoinTable() { Id = 1, Person_Id = 2, SomeOther_Id = 1000 },
    new JoinTable() { Id = 1, Person_Id = 2, SomeOther_Id = 2000 },
    new JoinTable() { Id = 1, Person_Id = 3, SomeOther_Id = 3000 },
    new JoinTable() { Id = 1, Person_Id = 3, SomeOther_Id = 4000 },
};

// Join the Table and select a new object.
var tmp = ps.Join(
    join, // which table/list should be joined
    o => o.Id, // key of outer list
    i => i.Person_Id, // key of inner list
    (o, i) => new { City = o.City, Id = i.Id, SomeOtherId = i.SomeOther_Id}); // form a new object with three properties..

// now we can group out new objects
var groupingByCity = tmp.GroupBy(g => g.City);

// let's see what we got..
foreach (var g in groupingByCity)
{
    Console.WriteLine(g.Key + ": " + g.Count());

    foreach (var g2 in g.GroupBy(a => a.SomeOtherId)) // And yes we can group out values again..
    {
        Console.WriteLine("    " + g2.Key + ": " + g2.Count());
    }
}

//示例数据（如果您能在问题中写一些就好了……）
List ps=新列表（）
{
new Person（）{Id=1，City=“Cologne”}，
新人（）{Id=2，City=“科隆”}，
新人（）{Id=3，City=“Berlin”}，
newperson（）{Id=4，City=“Berlin”}，
};
列表联接=新列表（）
{
新的JoinTable（）{Id=1，Person_Id=1，SomeOther_Id=1000}，
新的JoinTable（）{Id=1，Person_Id=1，SomeOther_Id=2000}，
新的JoinTable（）{Id=1，Person_Id=2，SomeOther_Id=1000}，
新的JoinTable（）{Id=1，Person_Id=2，SomeOther_Id=2000}，
新的JoinTable（）{Id=1，Person_Id=3，SomeOther_Id=3000}，
新的JoinTable（）{Id=1，Person_Id=3，SomeOther_Id=4000}，
};
//联接表并选择一个新对象。
var tmp=ps.Join(
join，//应该联接哪个表/列表
o=>o.Id，//外部列表的键
i=>i.Person\u Id，//内部列表的键
（o，i）=>new{City=o.City，Id=i.Id，SomeOtherId=i.SomeOther_Id}）；//形成一个具有三个属性的新对象。。
//现在我们可以将新对象分组
var groupingByCity=tmp.GroupBy（g=>g.City）；
//让我们看看我们得到了什么。。
foreach（groupingByCity中的var g）
{
Console.WriteLine（g.Key+：“+g.Count（））；
foreach（g.GroupBy中的var g2（a=>a.SomeOtherId））//是的，我们可以再次分组值。。
{
Console.WriteLine（“+g2.Key+”：“+g2.Count（））；
}
}

// Example Data (would be great if you could write some in your questions..)
List<Person> ps = new List<Person>()
{
    new Person() { Id = 1, City = "Cologne" },
    new Person() { Id = 2, City = "Cologne" },
    new Person() { Id = 3, City = "Berlin" },
    new Person() { Id = 4, City = "Berlin" },
};
List<JoinTable> join = new List<JoinTable>()
{
    new JoinTable() { Id = 1, Person_Id = 1, SomeOther_Id = 1000 },
    new JoinTable() { Id = 1, Person_Id = 1, SomeOther_Id = 2000 },
    new JoinTable() { Id = 1, Person_Id = 2, SomeOther_Id = 1000 },
    new JoinTable() { Id = 1, Person_Id = 2, SomeOther_Id = 2000 },
    new JoinTable() { Id = 1, Person_Id = 3, SomeOther_Id = 3000 },
    new JoinTable() { Id = 1, Person_Id = 3, SomeOther_Id = 4000 },
};

// Join the Table and select a new object.
var tmp = ps.Join(
    join, // which table/list should be joined
    o => o.Id, // key of outer list
    i => i.Person_Id, // key of inner list
    (o, i) => new { City = o.City, Id = i.Id, SomeOtherId = i.SomeOther_Id}); // form a new object with three properties..

// now we can group out new objects
var groupingByCity = tmp.GroupBy(g => g.City);

// let's see what we got..
foreach (var g in groupingByCity)
{
    Console.WriteLine(g.Key + ": " + g.Count());

    foreach (var g2 in g.GroupBy(a => a.SomeOtherId)) // And yes we can group out values again..
    {
        Console.WriteLine("    " + g2.Key + ": " + g2.Count());
    }
}