Linq
Linq语句
Linq查询表达式以from开头select或group结尾
2021-09-06
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from :指定查询操作的数据源和范围变量
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join :连接多个用于查询操作的数据源 , 输出相同的值
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into :提供一个临时标识符，join子句，group子句，或select子句均可以通过该标识符引用查询操作的中间结果
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let :引入用来临时保存查询表达式中的字表达式结果的范围变量
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where:指定筛选元素的逻辑条件
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orderby:对查询结果进行顺序操作，包括升序和降序
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select:指定查询结果的类型和表现形式
代码
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using System;
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using System.Linq;
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namespace Linq
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{
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class Program
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{
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static bool Bl(int a)
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{
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return a % 2 == 0 ? true : false;
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}
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static void Main(string[] args)
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{
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int[] Array = { 1, 2, 4, 5, 6, 7, 8 };
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var La = from a in Array
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select a;
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var La2 = from a in Array
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where a < 2
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select a;
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foreach (var a in La)
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Console.WriteLine("La = " + a);
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foreach (var a in La2)
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Console.WriteLine("La2 = " + a);
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Console.WriteLine("L2第一个元素 = " + La2.First());
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//两个数组相加，
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int[] A1 = { 1, 2, 3, 4, 5, 6, 7, 8 };
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int[] A2 = { 1, 2, 3, 4, 5, 6, 7, 8 };
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var Lin = from a1 in A1
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from a2 in A2
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select a1 + a2;
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foreach (var item in Lin)
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{
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Console.Write(item + " ");
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}
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//给结果值添加名称：i=1 ， name = 1
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int[] A3 = { 1, 2, 3, 4, 5, 6, 7, 8 };
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var Lin2 = from a1 in A3
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select new
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{
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i = a1,
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name = a1.ToString()
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};
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foreach (var L in Lin2)
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{
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Console.WriteLine("Lin2 = " + L);
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}
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//对每个值乘10
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int[] A4 = { 1, 2, 3, 4, 5, 6, 7, 8 };
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var Lin3 = from a in A4
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select a * 10;
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Console.WriteLine("Lin3 = " + Lin3.First());
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//where判断语句
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int[] A5 = { 1, 2, 3, 4, 5, 6, 7, 8 };
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var Lin4 = from a in A5
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where a > 7
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select a;
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//判断是偶数还是奇数,偶数显示奇数不显示
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int[] A6 = { 1, 2, 3, 4, 5, 6, 7, 8 };
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var Lin5 = from a in A6
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where Bl(a)
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select a;
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foreach (var i in Lin5)
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{
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Console.WriteLine("判断语句包含函数 = " + i);
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}
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//let语句， n = a % 2 （=偶数） ，where n == 0 （=偶数）
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int[] A7 = { 1, 2, 3, 4, 5, 6, 7, 8 };
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var Lin6 = from a in A7
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let n = a % 2
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where n == 0
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select a;
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foreach (var item in Lin6)
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{
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Console.WriteLine(" let语句 = " + item);
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}
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//降序或升序
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var Lin7 = from a in A7
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orderby a descending //ascending
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select a;
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foreach (var item in Lin7)
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{
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Console.WriteLine(" 升序或降序 = " + item);
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}
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//group by 分组语句
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var Lin8 = from a in A7
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group a by a % 2;
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foreach (var i in Lin8)
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{
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foreach (var j in i)
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{
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Console.WriteLine("group分组语句 = " + j);
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}
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}
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//group and into
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var Lin9 = from a in A7
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group a by a % 2 into Aa
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from InTo in Aa
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select InTo;
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foreach (var i in Lin9)
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{
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Console.WriteLine("group and into = " + i);
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}
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// join语句,连接两个数组，判断Array1和Array2相同的值
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int[] Array1 = { 1, 2, 3, 4, 5, 6, 7, 8, 9 };
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int[] Array2 = { 1, 78, 3, 4, 5, 84, 7, 8, 98 };
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var Query = from a in Array1
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join b in Array2 on a equals b
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select a;
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foreach (var i in Query)
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{
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Console.WriteLine("Join = "+i);
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}
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}
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}
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}
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深入_SelectMany(); 存在多个from语句时使用
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//常规方法-------------------------------------
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var Linq2 = from a in _Name
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from b in _Birth
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where a.height < b.Min_Height
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orderby a.Id
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select "姓名： " +a.Name+ " 年龄： "+a.Age + " 体重： "+a.Weight + " 身高： "+a.height + " 最低身高： "+b.Min_Height + " 最低体重 "+b.Max_Height + "\n";
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Console.WriteLine("不符合条件的有：");
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foreach (var item in Linq2)
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{
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Console.WriteLine(item);
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}
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//SelectMany() --------使用方式------------------------------
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var Linq2_J = _Name.SelectMany(B => _Birth, (N, B) => new { n = N, b = B })
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.Where(N => N.n.height < N.b.Min_Height)
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.OrderBy(N => N.n.Id)
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.Select(N=> "姓名： " + N.n.Name + " 年龄： " + N.n.Age + " 体重： " + N.n.Weight + " 身高： " + N.n.height + " 最低身高： " + N.b.Min_Height + " 最低体重 " + N.b.Max_Height + "\n");
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Console.WriteLine("不符合条件的有：");
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foreach (var item in Linq2_J)
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{
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Console.WriteLine(item);
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}
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//方法1-----------------------------------
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var i = 1;
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var Linq3 = from a in _Name
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from b in _Birth
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where a.Kungfu == b.Kungfu
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orderby a.ko * b.ko descending, a.Age, a.Name
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select "Id: " + a.Id + " 姓名： " + a.Name + " 所练功夫： " + a.Kungfu + " 排位： " + (i++);
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Console.WriteLine("全武林武力值排行榜：");
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foreach (var item in Linq3)
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{
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Console.WriteLine(item);
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}
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//方法2----------------------------------------------------------
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i = 1;
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var Linq3_J = _Name.SelectMany(B => _Birth, (N, B) => new { n = N, b = B })
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.Where(C => C.n.Kungfu == C.b.Kungfu)
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.OrderByDescending(C => C.n.ko * C.b.ko)
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.ThenBy(C => C.n.Name)
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.Select(C=>"ID= "+ C.n.Id+" 姓名： "+C.n.Name+" 所练功夫： "+C.n.Kungfu+"武力值：" + (C.n.ko * C.b.ko) + " 排位: "+(i++));
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Console.WriteLine("全武林武力值排行榜：");
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foreach (var item in Linq3_J)
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{
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Console.WriteLine(item);
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}
SelectMany()
Join语句
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int J = 1;
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var Linq4 = from a in _Birth
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where a.ko > 60
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orderby a.ko descending
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select new {id = a.Id,kungfu = a.Kungfu,ko=a.ko,top = (J++) };
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int Ji = 1;
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var Linq4_J = _Name.Where(b => b.ko > 6)
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.OrderByDescending(b => b.ko)
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.Select(b=>new {id=b.Id,name = b.Name,ko = b.ko ,kungfu = b.Kungfu ,top = (Ji++) });
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//连接两个对象进行比较胜负
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int Jii = 1;
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var Linq5 = from a in Linq4
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join b in Linq4_J on a.kungfu equals b.kungfu
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orderby a.ko * b.ko descending
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select new { id = b.id, name = b.name, ko = (a.ko * b.ko)
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,kungfu = a.kungfu,Birth = a.top, name_top = b.top, top = Jii++};
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//方法二：使用拓展方法
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int Jiii = 1;
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var Linq5_J = Linq4.Join(Linq4_J, a => a.kungfu, b => b.kungfu, (A, B) =>
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new { id = B.id, name = B.name, ko = (B.ko * A.ko), kungfu = A.kungfu,
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Birth = A.top, name_top = B.top, top = (Jiii++) })
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.OrderByDescending(m=>m.ko);
Join
对武林功夫进行排序,此方法是透过count计数排序,判断该武功有多少人使用，从而做到分组
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//Linq
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var Linq6 = from a in _Birth
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join b in _Name on a.Kungfu equals b.Kungfu into Groups
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orderby Groups.Count() descending
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select new {Id = a.Id , Kungfu = a.Kungfu , ko = a.ko ,count = Groups.Count() };
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Console.WriteLine("功夫使用人数分组：");
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foreach (var item in Linq6)
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{
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Console.WriteLine("ID: "+item.Id+" 功夫： "+item.Kungfu+" 武力值： "+item.ko +" 此武功使用人数： "+item.count );
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}
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//拓展方法
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var Linq6_J = _Birth.GroupJoin(_Name, A => A.Kungfu, B => B.Kungfu, (A, B) => new { A.Id, A.Kungfu, A.ko, count = B.Count() })
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.OrderByDescending(C=>C.count);
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Console.WriteLine("功夫使用人数分组：");
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foreach (var item in Linq6_J)
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{
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Console.WriteLine("ID： "+item.Id+" 功夫： "+item.Kungfu+" 武力值： "+item.ko+" 此功夫使用人数： "+item.count );
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}
GroupJoin()
对武林武功进行排序，此方法是透过id安id大小排序做到分组
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//Linq语句
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var Linq7 = from a in _Birth
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join b in _Name on a.Kungfu equals b.Kungfu
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orderby a.Id
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select new { Id = a.Id, name = b.Name, Kungfu = a.Kungfu, ko = (a.ko * b.ko) };
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Console.WriteLine("joinorGroup分组");
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foreach (var item in Linq7)
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{
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Console.WriteLine("Id= " + item.Id + " 姓名： " + item.name + " 所练功夫： " + item.Kungfu + " 武力值： " + item.ko);
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}
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//拓展方法
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var Linq7_J = _Birth.GroupJoin(_Name, A => A.Kungfu, B => B.Kungfu, (A, B) => new { Id = A.Id, count = B.Count(), Kungfu = A.Kungfu })
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.OrderBy(m=>m.Id);
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Console.WriteLine("joinorGroup分组");
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foreach (var item in Linq7_J)
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{
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Console.WriteLine("Id= " + item.Id + " 所练功夫： " + item.Kungfu+" 使用人数： "+item.count);
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}
Group分组武功，linq与Groupby();
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//linq语句
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var Linq8 = from a in _Name
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group a by a.Kungfu into b
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orderby b.Key
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select new {kungfu = b.Key,count = b.Count() };
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Console.WriteLine("分组个各武功使用数");
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foreach (var item in Linq8)
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{
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Console.WriteLine("功夫名： "+item.kungfu+" 使用人数： "+item.count);
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}
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//拓展方法
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var Lin8_J = _Name.GroupBy(a=>a.Kungfu,(A,B)=>new {kungfu =A,count = B.Count() })
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.OrderBy(m=>m.kungfu);
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Console.WriteLine("分组个各武功使用数");
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foreach (var item in Lin8_J)
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{
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Console.WriteLine("功夫名： " + item.kungfu + " 使用人数： " + item.count);
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}
判断对象里面是否包含指定内容 变量.Any();
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bool lbwb = Lin8_J.Any(v=>v.kungfu == "凌波微步");//判断对象是否包含用凌波微步的人
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Console.WriteLine("是否含有使用凌波微步的人："+lbwb);//返回布尔值
判断是否所有对象都一致： 变量.All();
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bool xlsbz = Lin8_J.All(v=>v.kungfu == "降龙十八掌");
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Console.WriteLine("所有人都使用降龙十八掌："+xlsbz);
判断对象里面是否包含某个对象
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var QiaoFeng = _Name[5];
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bool IsQiaoFeng = _Name.Contains(QiaoFeng);//判断某对象是否存在
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Console.WriteLine("江湖中是否存在乔峰这个人："+IsQiaoFeng);
算出对象总数和页码
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分页
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double Page = 10;//每页数量
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int name_count = _Name.Count();//对象总数
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double count = Math.Ceiling(name_count / Page);//计算总共有多少页
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Console.WriteLine("当前对象总页数："+count+" 当前对象总数："+ name_count);
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var Linq9 = (from a in _Name
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join b in _Birth on a.Kungfu equals b.Kungfu
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orderby a.ko * b.ko descending
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select new { name = a.Name, Kungfu = b.Kungfu, ko = a.ko * b.ko })
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.Skip(0 * (int)Page) // 0 *Page = 第一页，以此类推
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.Take((int)Page); // 返回指定数量（Page）元素
对比两个值返回值相同的集合
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//如果功夫为 降龙十八掌 和武力值为 10 就能返回
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var Linq10 =(from a in _Name where a.Kungfu == "降龙十八掌" select a)
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.Intersect(from b in _Name where b.ko == 10 select b);//Intersect:返回比较值相同的集合
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Console.WriteLine("\n比较两个值返回值相同的集合：");
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foreach (var item in Linq10)
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{
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Console.WriteLine(" 姓名 "+item.Name+" 功夫 "+item.Kungfu +" 等级 "+item.ko);
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}
累加：Aggregate
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int[] array = { 1,2,3,4,5,6};
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int arr1 = array.Aggregate((a,b)=>a+b); //Aggregate累加 最后结果=21
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int arr2 = array.Aggregate(0,(a,b)=>a+b , r=>r*2); //Aggregate累加 最后结果=42
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int arr3 = array.Aggregate(9,(a,b)=>a+b); //Aggregate累加 最后结果=30
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Console.WriteLine(arr1 +" "+ arr2 + " " + arr3);
linq模糊查询
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1.判断是否为空或者null
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string.IsNullOrEmpty(des.PlateNum)————————>sql server的PlateNum is null的判断
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from des in db.ModelsVehicleRecognition where (!string.IsNullOrEmpty(des.PlateNum)) select new { plateMun = des.PlateNum }
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等同于
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SELECT PlateNum FROM VehicleRecognition WHERE PlateNum IS NOT NULL
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2.普通包含模糊查询
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1）以某字符串开头的模糊查询
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des.PlateNum.StartsWith("皖A8") ————————>sql server 的 like '皖A8%'
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from des in db.ModelsVehicleRecognition where (des.PlateNum.StartsWith("皖A8")) select new { plateMun = des.PlateNum }
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等同于
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SELECT PlateNum FROM VehicleRecognition WHERE PlateNum like '皖A8%'
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2）以某字符串结尾的模糊查询
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des.PlateNum.EndsWith("68T") ————————>sql server 的 like '%68T'
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from des in db.ModelsVehicleRecognition where (des.PlateNum.EndsWith("68T")) select new { plateMun = des.PlateNum }
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等同于
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SELECT PlateNum FROM VehicleRecognition WHERE PlateNum like '%68T'
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3）包含某字符串的模糊查询
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des.PlateNum.Contains("A3") ————————>sql server 的 like '%A3%'
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from des in db.ModelsVehicleRecognition where (des.PlateNum.Contains("A3")) select new { plateMun = des.PlateNum }
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等同于
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SELECT PlateNum FROM VehicleRecognition WHERE PlateNum like '%A3%'
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3.精确到字符串对应位数字符的模糊查询（*重点）
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SqlFunctions.PatIndex("_a__3%", des.PlateNum) > 0————————>sql server 的 like '_a__3%'
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from des in db.ModelsVehicleRecognition where (SqlFunctions.PatIndex("_a__3%", des.PlateNum) > 0) select new { plateMun = des.PlateNum }
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等同于
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SELECT PlateNum FROM VehicleRecognition WHERE PlateNum like '_a__3%'
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说明：'_a__3%' 中的下划线“_”表示一个字符，'_a__3%' 这个字符串查询意思就是第二个字符是a,第五个字符是3的字符串
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因为a和3之间有两个下划线“_”所以查询出的结果也要满足a和3之间有两个字符才行，
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也就是说两个精确字符之间隔了几个字符，在查询的时候就要写几个下划线“_”。
本文来自博客园，作者：Entity110，转载请注明原文链接：https://www.cnblogs.com/rdr2/p/15232089.html
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