LINQ查询表达式必须以from子句开头,以select或group子句结束ide
from...in...:指定要查找的数据以及范围变量,多个from子句则表示从多个数据源查找数据。注意:C#编译器会把"复合from子句"的查询表达式转换为SelectMany()扩展方法学习
join...in...on...equals...:指定多个数据源的关联方式spa
let:引入用于存储查询表达式中子表达式结果的范围变量,一般能达到层次感会更好,使代码更易于月的翻译
orderby、descending:指定元素的排序字段和排序方式,当有多个排序字段时,由字段顺序肯定主次关系,可指定升序和降序两种排序方式code
where:指定元素的筛选条件,多个where子句则表示了并列条件,必须所有都知足才能入选,每一个where子句可使用&&、||链接多个条件表达式orm
group:指定元素的分组字段blog
select:指定查询要返回的目标数据,能够指定任何类型,甚至是匿名类型(目前一般被指定为匿名类型)排序
into:提供一个临时的标识符,该标识符能够引用join、group和select子句的结果。(1)直接出如今join子句以后的into关键字会被翻译为GroupJoin。(2)select或group子句字后的into它会从新开始一个查询,让咱们能够继续引入where、orderby和select子句,它是对分步构建查询表达式的一种简写方式。get
下面经过一个案例来学习对两张表进行查询编译器
using System; using System.Collections.Generic; using System.Linq; using System.Text; using System.Threading.Tasks; namespace _013_LINQ { /// <summary> /// 武林高手 /// </summary> class MartialArtsMaster { public int Id { get; set; } public string Name { get; set; } public int Age { get; set; } public string Menpai { get; set; } public string Kongfu { get; set; } public int Level { get; set; } public override string ToString() { return string.Format("Id: {0}, Name: {1}, Age: {2}, Menpai: {3}, Kongfu: {4}, Level: {5}", Id, Name, Age, Menpai, Kongfu, Level); } } }
using System; using System.Collections.Generic; using System.Linq; using System.Text; using System.Threading.Tasks; namespace _013_LINQ { /// <summary> /// 武学 /// </summary> class Kongfu { public int Id { get; set; } public string Name { get; set; } public int Power { get; set; } public override string ToString() { return string.Format("Id: {0}, Name: {1}, Power: {2}", Id, Name, Power); } } }
using System; using System.Collections.Generic; using System.Linq; using System.Security.Cryptography; using System.Text; using System.Threading.Tasks; namespace _013_LINQ { internal class Program { private static void Main(string[] args) { //初始化武林高手 var masterList = new List<MartialArtsMaster>() { new MartialArtsMaster() {Id = 1, Name = "黄蓉", Age = 18, Menpai = "丐帮", Kongfu = "打狗棒法", Level = 9}, new MartialArtsMaster() {Id = 2, Name = "洪七公", Age = 70, Menpai = "丐帮", Kongfu = "打狗棒法", Level = 10}, new MartialArtsMaster() {Id = 3, Name = "郭靖", Age = 22, Menpai = "丐帮", Kongfu = "降龙十八掌", Level = 10}, new MartialArtsMaster() {Id = 4, Name = "任我行", Age = 50, Menpai = "明教", Kongfu = "葵花宝典", Level = 1}, new MartialArtsMaster() {Id = 5, Name = "东方不败", Age = 35, Menpai = "明教", Kongfu = "葵花宝典", Level = 10}, new MartialArtsMaster() {Id = 6, Name = "林平之", Age = 23, Menpai = "华山", Kongfu = "葵花宝典", Level = 7}, new MartialArtsMaster() {Id = 7, Name = "岳不群", Age = 50, Menpai = "华山", Kongfu = "葵花宝典", Level = 8}, new MartialArtsMaster() {Id = 8, Name = "令狐冲", Age = 23, Menpai = "华山", Kongfu = "独孤九剑", Level = 10}, new MartialArtsMaster() {Id = 9, Name = "梅超风", Age = 23, Menpai = "桃花岛", Kongfu = "九阴真经", Level = 8}, new MartialArtsMaster() {Id = 10, Name = "黄药师", Age = 23, Menpai = "梅花岛", Kongfu = "弹指神通", Level = 10}, new MartialArtsMaster() {Id = 11, Name = "风清扬", Age = 23, Menpai = "华山", Kongfu = "独孤九剑", Level = 10} }; //初始化武学 var kongfuList = new List<Kongfu>() { new Kongfu() {Id = 1, Name = "打狗棒法", Power = 90}, new Kongfu() {Id = 2, Name = "降龙十八掌", Power = 95}, new Kongfu() {Id = 3, Name = "葵花宝典", Power = 100}, new Kongfu() {Id = 4, Name = "独孤九剑", Power = 100}, new Kongfu() {Id = 5, Name = "九阴真经", Power = 100}, new Kongfu() {Id = 6, Name = "弹指神通", Power = 100} }; }
上面定义了两个类,武林高手和武学,如今咱们查询全部武学级别大于8且门派为丐帮的武林高手。
首先咱们使用原始的方法来查询:
{ var res = new List<MartialArtsMaster>(); foreach (var temp in masterList) { if (temp.Level > 8 && temp.Menpai == "丐帮") { res.Add(temp); }
//1,使用LINQ作查询( 表达式写法) var res = from m in masterList //from后面设置查询的集合 where m.Level > 8 && m.Menpai == "丐帮" //where后面跟上查询的条件 select m;//表示m的结果结合返回
//过滤方法 static bool Test1(MartialArtsMaster master) { if (master.Level > 8) return true; return false; } //2,扩展方法的写法 //var res = masterList.Where(Test1); var res = masterList.Where(m => m.Level > 8 && m.Menpai == "丐帮");
查询结果:
LINQ也能够同时查多个表,下面咱们取得所学功夫的杀伤力大于90 的武林高手。
var res = from m in masterList from k in kongfuList where m.Kongfu == k.Name && k.Power > 90 select m;
var res = masterList.SelectMany(m => kongfuList, (m, k) => new { master = m, kongfu = k }) .Where(x => x.master.Kongfu == x.kongfu.Name && x.kongfu.Power > 90);
查询结果:
优先等级排序,而后年龄排序
var res = from m in masterList where m.Level > 8 && m.Menpai == "丐帮" //orderby m.Age descending // 默认从小到大,加上descending从大到小 orderby m.Level, m.Age //按照多个字段进行排序,若是字段的属性相同,就按照第二个属性排序 select m;//表示m的结果结合返回
var res = masterList.Where(m => m.Level > 8 && m.Menpai == "丐帮").OrderBy(m => m.Age);
若是排序判断条件有多个,后面的排序要用ThenBy
var res = masterList.Where(m => m.Level > 8).OrderBy(m => m.Level).ThenBy(m => m.Age);
查询结果:
var res = from m in masterList // join...in... 表示要链接的表,on后面为链接条件,等于要用equals,不能用== join k in kongfuList on m.Kongfu equals k.Name where k.Power > 90 select new { master = m, kongfu = k };
查询结果:
把武林高手按照所学功夫分类,看一下那个功夫修炼的人数最多
var res = from k in kongfuList join m in masterList on k.Name equals m.Kongfu into groups //分组 orderby groups.Count() // 这个能够得到数量 select new { kongfu = k, count = groups.Count() };
查询结果:
var res = from m in masterList group m by m.Kongfu into g select new { count = g.Count(), key = g.Key };//g.Key Key表示是按照那个属性分的组
结果:
any判断集合中是否有一个知足,all判断集合中是否所有知足
bool res = masterList.Any(m => m.Menpai == "长留"); Console.WriteLine(res); res = masterList.All(m => m.Menpai == "丐帮"); Console.WriteLine(res);
结果: