指的是结果生成两张表可以连接的部分
private void button1_Click_1(object sender, EventArgs e)
{
//初始化Student数组
Student[] arrStu = new Student[]{
new Student{ID=1,SName="zhangsan",Age=20},
new Student{ID=2,SName="lisi",Age=21},
new Student{ID=3,SName="wangwu",Age=23},
new Student{ID=4,SName="liuliu",Age=24},
};
//初始化Product数组
Product[] arrPro = new Product[]{
new Product{ID=1,PName="Apple",Price=2.25},
new Product{ID=2,PName="Orange",Price=5.25},
new Product{ID=3,PName="Banana",Price=7.5},
new Product{ID=4,PName="StrawBerry",Price=6.5},
};
//标准查询运算符
var query = arrStu.Join(arrPro, s => s.ID, p => p.ID, (s, p) =>new { s.SName, s.Age, p.PName, p.Price });
StringBuilder sbRes = new StringBuilder();
//打印
foreach (var item in query)
{
sbRes.AppendFormat("SName:{0},Age:{1},PName:{2},Price:{3}", item.SName, item.Age, item.PName, item.Price);
sbRes.AppendLine();
}
MessageBox.Show(sbRes.ToString());
}
或者另外一种写法
//LINQ语句
var query = from sItem in arrStu
join pItem in arrPro
on sItem.ID equals pItem.ID
select new { sItem.SName, sItem.Age, pItem.PName, pItem.Price };//Select后面接一个匿名对象
第一个集合中的每个元素与第二个集合中的一组相关元素进行配对
使用 into关键词
Person magnus = new(FirstName: "Magnus", LastName: "Hedlund");
Person terry = new("Terry", "Adams");
Person charlotte = new("Charlotte", "Weiss");
Person arlene = new("Arlene", "Huff");
List people = new() { magnus, terry, charlotte, arlene };
List pets = new()
{
new(Name: "Barley", Owner: terry),
new("Boots", terry),
new("Whiskers", charlotte),
new("Blue Moon", terry),
new("Daisy", magnus),
};
// Create a list where each element is an anonymous type
// that contains the person's first name and a collection of
// pets that are owned by them.
var query =
from person in people
join pet in pets on person equals pet.Owner into gj
select new
{
OwnerName = person.FirstName,
Pets = gj
};
foreach (var v in query)
{
// Output the owner's name.
Console.WriteLine($"{v.OwnerName}:");
// Output each of the owner's pet's names.
foreach (var pet in v.Pets)
{
Console.WriteLine($" {pet.Name}");
}
}
/* Output:
Magnus:
Daisy
Terry:
Barley
Boots
Blue Moon
Charlotte:
Whiskers
Arlene:
*/
3. 左外连接
简单理解就是,基于第一张表,无论第二章表是否有匹配项,都放进去
左连接的数据 一定 大于等于 内连接
Person magnus = new("Magnus", "Hedlund");
Person terry = new("Terry", "Adams");
Person charlotte = new("Charlotte", "Weiss");
Person arlene = new("Arlene", "Huff");
Pet barley = new("Barley", terry);
Pet boots = new("Boots", terry);
Pet whiskers = new("Whiskers", charlotte);
Pet bluemoon = new("Blue Moon", terry);
Pet daisy = new("Daisy", magnus);
// Create two lists.
List people = new() { magnus, terry, charlotte, arlene };
List pets = new() { barley, boots, whiskers, bluemoon, daisy };
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 ?? string.Empty
};
foreach (var v in query)
{
Console.WriteLine($"{v.FirstName + ":",-15}{v.PetName}");
}
record class Person(string FirstName, string LastName);
record class Pet(string Name, Person Owner);
// This code produces the following output:
//
// Magnus: Daisy
// Terry: Barley
// Terry: Boots
// Terry: Blue Moon
// Charlotte: Whiskers
// Arlene:
reference:
1. 执行左外部联接(C# 中的 LINQ) - C# | Microsoft Learn