外部连接两个具有空值的表

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; }
}

和此数据：

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}
};

现在我想得到所有人的名单和他们的宠物名字（如果没有宠物，则为空）结果应该是

Foo - FooBoy
Bar - null

我试过了

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 上收到空引用异常。如果我删除 linkPets.DefaultIfEmpty()，我只会得到第一个人。

Answer 1

题改前

（最初 link table 引用了宠物，而不是 ID。）

在我看来你只需要一个连接，因为你可以只使用 link 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 };

改题后

我保留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 之间进行连接即可：

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，你可以在一条语句中完成，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

如果你想用 link table 同时获得人和宠物，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

另一种解决方案是使用 linkPets.DefaultIfEmpty(new Link())（可能只适用于内存中的联接，因为您的原始联接适用于数据库）

 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) };

外部连接两个具有空值的表

Outer join two tables with null values

c#

linq

outer-join