最近在优化定时任务相关的代码，建议是把总查询放到内存中去坐，尽量减少打开的数据库连接

1. 内连接

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 arrStujoin pItem in arrProon sItem.ID equals pItem.IDselect new { sItem.SName, sItem.Age, pItem.PName, pItem.Price };//Select后面接一个匿名对象 

2.  分组连接

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<Person> people = new() { magnus, terry, charlotte, arlene };List<Pet> 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 peoplejoin pet in pets on person equals pet.Owner into gjselect 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:DaisyTerry:BarleyBootsBlue MoonCharlotte:WhiskersArlene:*/

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<Person> people = new() { magnus, terry, charlotte, arlene };
List<Pet> pets = new() { barley, boots, whiskers, bluemoon, daisy };var query =from person in peoplejoin pet in pets on person equals pet.Owner into gjfrom 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