[英]C# Linq Inner Join
我想选择只有宠物的人。
当我执行查询
var query = from p in people
join
pts in pets
on p equals pts.Owner into grp
select new {grp=grp,PersonName=p.FirstName};
没有宠物的人也会被选中。
我的名单是
Person[] prn = new Person[3];
prn[0] = new Person();
prn[0].FirstName = "Jon";
prn[0].LastName = "Skeet";
prn[1] = new Person();
prn[1].FirstName = "Marc";
prn[1].LastName = "Gravell";
prn[2] = new Person();
prn[2].FirstName = "Alex";
prn[2].LastName = "Grover";
List<Person> people = new List<Person>();
foreach (Person p in prn)
{
people.Add(p);
}
Pet[] pt = new Pet[3];
pt[0] = new Pet();
pt[0].Name = "Zonny";
pt[0].Owner = people[0];
pt[1] = new Pet();
pt[1].Name = "Duggie";
pt[1].Owner = people[0];
pt[2] = new Pet();
pt[2].Name = "Zoggie";
pt[2].Owner = people[1];
List<Pet> pets=new List<Pet>();
foreach(Pet p in pt)
{
pets.Add(p);
}
那是因为你正在使用join ... into
群组加入。 你只想要一个普通的连接:
var query = (from p in people
join pts in pets on p equals pts.Owner
select p).Distinct();
或者,如果您想要有宠物的人及其主人,您可以做以下事情:
var query = pets.GroupBy(pet => pet.Owner)
.Select(x => new { Owner = x.Key, Pets = x.ToList() });
这将产生一个结果,你可以得到每个主人和他们的宠物,但只有有宠物的人。
如果你想要别的东西,请告诉我们......
顺便说一下,现在是了解对象和集合初始化器的好时机。 这是一种初始化people
列表的简单方法,例如:
List<Person> people = new List<Person>
{
new Person { FirstName = "Jon", LastName = "Skeet" },
new Person { FirstName = "Marc", LastName = "Gravell" },
new Person { FirstName = "Alex", LastName = "Grover" },
};
更紧凑:)
编辑:交叉连接很简单:
var query = from person in people
from pet in pets
select new { person, pet };
使用组连接有效地模拟左连接。 听起来你有深度C#,我建议你仔细阅读第11章:)
这是一种不同的方法,只添加一行:
var query = from p in people
join pts in pets
on p equals pts.Owner into grp
where grp.Any() // <--- added this
select new {grp=grp,PersonName=p.FirstName};
在这里,我像你一样选择组,但我添加了一行,只选择包含至少一个元素的组,并忽略其余的组。
这也可以在单行代码中使用lambda表达式完成...
IEnumerable<Person> peopleWithPets = people.Where(x => pets.Any(y => y.Owner == x));
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