select

  cl.Name as clientName,

  ci.Name as cityName

from Client cl

  left join City ci on ci.ID = cl.CityID

using System;

using System.Collections.Generic;

using System.Linq;

 

namespace TestLinq

{

  class Program

  {

    public class Client

    {

      public int ID;

      public string Name;

      public int ?CityID;

    }

 

    public class City

    {

      public int ID;

      public string Name;

      public int ?RegionID;

    }

 

    public class Region

    {

      public int ID;

      public string Name;

    }

 

    public static List<Client> GetClients()

    {

      return new List<Client>()

      {

        new Client {ID = 1, Name = "Client A", CityID = 1 },

        new Client {ID = 2, Name = "Client B", CityID = 2 },

        new Client {ID = 3, Name = "Client C", CityID = null },

        new Client {ID = 4, Name = "Client D", CityID = 2 }

      };

    }

 

    public static List<City> GetCities()

    {

      return new List<City>()

      {

        new City {ID = 1, Name = "City A", RegionID = 1 },

        new City {ID = 2, Name = "City B", RegionID = 3 },

        new City {ID = 3, Name = "City C", RegionID = null },

        new City {ID = 4, Name = "City D", RegionID = 3 }

      };

    }

 

    public static List<Region> GetRegions()

    {

      return new List<Region>()

      {

        new Region {ID = 1, Name = "Region A"},

        new Region {ID = 2, Name = "Region B"},

        new Region {ID = 3, Name = "Region C"},

        new Region {ID = 4, Name = "Region D"}

      };

    }

 

    static void Main(string[] args)

    {

      var result =

        from clients in GetClients()

        join cities in GetCities() on clients.CityID equals cities.ID into clientCityGroup

        from subCities in clientCityGroup.DefaultIfEmpty()

        select new { clientName = clients.Name, cityName = subCities?.Name };

 

      foreach (var current in result)

        Console.WriteLine($"{current.clientName} из города {current?.cityName}");

 

      Console.ReadKey();

    }

  }

}

select

  cl.Name as clientName,

  ci.Name as cityName,

  rg.Name as regionName

from Client cl

  left join City ci   on ci.ID = cl.CityID

  left join Region rg on rg.ID = ci.RegionID