Question

我有这个数据结构：

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

class Pet
{
    public int Id { get; set; }
    public string Name { get; set; }
}

class Link
{
    public Person Person { get; set; }
    public int PetId { get; set; }
}

和这个数据：

List<Person> people = new List<Person>
{
    new Person {FirstName = "Foo"},
    new Person {FirstName = "Bar"}
};
List<Pet> pets = new List<Pet>
{
    new Pet {Id = 1, Name = "FooBoy"},
};
List<Link> links = new List<Link>
{
    new Link {Person = people.First(), PetId = pets.First().Id}
};

现在我要获取所有人及其宠物名称的列表（如果没有宠物，则为null）结果应该是

Foo - FooBoy
Bar - null

我试过

var query = from person in people
    join lnk in links on person equals lnk.Person into linkPets
    from link in linkPets.DefaultIfEmpty()
    join p in pets on link.PetId equals p.Id into subPets
    from subPet in subPets.DefaultIfEmpty()
    select new { person.FirstName, PetName = (subPet == null ? String.Empty : subPet.Name) };

但我在link.Pet上得到空引用异常。如果我删除linkPets.DefaultIfEmpty()，我只会获得第一个人。

Answer 1

在问题发生变化之前

（最初链接表引用了宠物，而不是ID。）

在我看来，你只需要一个连接，因为你可以使用链接中的Pet属性，其中有一个：

var query = from person in people
            join lnk in links on person equals lnk.Person into linkPets
            from link in linkPets.DefaultIfEmpty()
            select new { person.FirstName,
                         PetName = link == null ? "" : link.Pet.Name };

问题改变后

我已经离开了上面的原始解决方案，因为很容易将修改后的问题转换为原始问题 - 只需先在pets和links之间进行联接：

var fullLinks = from link in links
                join pet in pets on link.PetId equals pet.Id
                select new { link.Person, Pet = pet };

var query = from person in people
    join lnk in fullLinks on person equals lnk.Person into linkPets
    from link in linkPets.DefaultIfEmpty()
    select new { person.FirstName,
                 PetName = link == null ? "" : link.Pet.Name };

如果你愿意，可以在一个声明中完成，但我不会：

var query = from person in people
    join lnk in (from link in links
                 join pet in pets on link.PetId equals pet.Id
                 select new { link.Person, Pet = pet } )
      on person equals lnk.Person into linkPets
    from link in linkPets.DefaultIfEmpty()
    select new { person.FirstName,
                 PetName = link == null ? "" : link.Pet.Name };

Answer 2

如果你想通过链接表获得人和宠物，你可以这样做：

var result= (
        from person in people
        from lnk in links
            .Where (w =>w.Person==person).DefaultIfEmpty()
        from p in pets
            .Where (w =>w==(lnk==null?null:lnk.Pet)).DefaultIfEmpty()
        select new { person.FirstName, PetName = (p == null ? String.Empty : p.Name) }
    ).Dump();

Answer 3

另一个解决方案是使用linkPets.DefaultIfEmpty（new Link（））（可能只适用于加入内存，因为您的原始加入适用于数据库）

 var query = from person in people
                   join lnk in links on person equals lnk.Person into linkPets
                   from link in linkPets.DefaultIfEmpty(new Link())
                   join p in pets on link.PetId equals p.Id into subPets
                   from subPet in subPets.DefaultIfEmpty()
                   select new { person.FirstName, PetName = (subPet == null ? String.Empty : subPet.Name) };

外连接两个具有空值的表

3 个答案: