C#の概念 LINQ（内部結合）

　SQLと同じJOIN句。

成果物

• github

情報源

• https://docs.microsoft.com/ja-jp/dotnet/csharp/linq/perform-inner-joins

コード

using System;
using System.Collections.Generic;
using System.Linq;

class Person
{
    public string Name { get; set; }
}
class Pet
{
    public string Name { get; set; }
    public Person Owner { get; set; }
}
class Main {
    private List<Person> persons;
    private List<Pet> pets;
    public void Run() {
        persons = CreatePersons();
        pets = CreatePets();
        Show(Query());
    }
    private List<Person> CreatePersons() {
        return new List<Person>() {
            new Person { Name="A" },
            new Person { Name="B" },
        };
    }
    private List<Pet> CreatePets() {
        return new List<Pet>() {
            new Pet { Name="a", Owner=persons[0] },
            new Pet { Name="b", Owner=persons[0] },
            new Pet { Name="c", Owner=persons[1] },
        };
    }
    private IEnumerable<dynamic> Query() {
        return  from person in persons
                join pet in pets on person equals pet.Owner
                select new { OwnerName=person.Name, PetName=pet.Name};
    }
    private void Show(in IEnumerable<dynamic> query) {
        foreach (var item in query) {
            Console.WriteLine($"OwnerName={item.OwnerName}, PetName={item.PetName}");
        }
    }
}

OwnerName=A, PetName=a
OwnerName=A, PetName=b
OwnerName=B, PetName=c

所感

　ポイントはjoin。

from person in persons
join pet in pets on person equals pet.Owner
select new { OwnerName=person.Name, PetName=pet.Name };

　SQLで書くと以下。

SELECT
  person.Name,
  pet.Name
FROM
  Persons person
  INNER JOIN Pets pet 
  ON person.id = pet.owner_id;

　LINQはid列が不要。そのままインスタンスで指定できるので読みやすい。

対象環境

• 2019-12-05
• Raspbierry pi 3 Model B+
• Raspbian stretch 9.0 2018-11-13 ※
• bash 4.4.12(1)-release ※
• SQLite 3.29.0 ※
• C# dotnet 3.0.100 ※

$ uname -a
Linux raspberrypi 4.19.42-v7+ #1218 SMP Tue May 14 00:48:17 BST 2019 armv7l GNU/Linux