Left Outer Join

class Person
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
}

class Pet
{
    public string Name { get; set; }
    public Person Owner { get; set; }
}

public static void Main(string[] args)
{
    var magnus = new Person { FirstName = "Magnus", LastName = "Hedlund" };
    var terry = new Person { FirstName = "Terry", LastName = "Adams" };

    var barley = new Pet { Name = "Barley", Owner = terry };

    var people = new[] { magnus, terry };
    var pets = new[] { barley };

    var query =
        from person in people
        join pet in pets on person equals pet.Owner into gj
        from subpet in gj.DefaultIfEmpty()
        select new
        {
            person.FirstName,
            PetName = subpet?.Name ?? "-" // Use - if he has no pet
        };

    foreach (var p in query)
        Console.WriteLine($"{p.FirstName}: {p.PetName}");
}