LINQ查詢表示式

約束

LINQ查詢表示式必須以from子句開頭，以select或group子句結束

關鍵字

from...in...：指定要查詢的資料以及範圍變數，多個from子句則表示從多個數據源查詢資料。注意：C#編譯器會把"複合from子句"的查詢表示式轉換為SelectMany()擴充套件方法

join...in...on...equals...：指定多個數據源的關聯方式

let：引入用於儲存查詢表示式中子表示式結果的範圍變數，通常能達到層次感會更好，使程式碼更易於月的

orderby、descending：指定元素的排序欄位和排序方式，當有多個排序欄位時，由欄位順序確定主次關係，可指定升序和降序兩種排序方式

where：指定元素的篩選條件，多個where子句則表示了並列條件，必須全部都滿足才能入選，每個where子句可以使用&&、||連線多個條件表示式

group：指定元素的分組欄位

select：指定查詢要返回的目標資料，可以指定任何型別，甚至是匿名型別（目前通常被指定為匿名型別）

into：提供一個臨時的識別符號，該識別符號可以引用join、group和select子句的結果。(1)直接出現在join子句之後的into關鍵字會被翻譯為GroupJoin。(2)select或group子句字後的into它會重新開始一個查詢，讓我們可以繼續引入where、orderby和select子句，它是對分步構建查詢表示式的一種簡寫方式。

下面通過一個案例來學習對兩張表進行查詢

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>
    /// 武學
    /// </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 = "黃蓉", Age = 18, Menpai = "丐幫", Kongfu = "打狗棒法", Level = 9},
                new MartialArtsMaster() {Id = 2, Name = "洪七公", Age = 70, Menpai = "丐幫", Kongfu = "打狗棒法", Level = 10},
                new MartialArtsMaster() {Id = 3, Name = "郭靖", Age = 22, Menpai = "丐幫", Kongfu = "降龍十八掌", Level = 10},
                new MartialArtsMaster() {Id = 4, Name = "任我行", Age = 50, Menpai = "明教", Kongfu = "葵花寶典", Level = 1},
                new MartialArtsMaster() {Id = 5, Name = "東方不敗", Age = 35, Menpai = "明教", Kongfu = "葵花寶典", Level = 10},
                new MartialArtsMaster() {Id = 6, Name = "林平之", Age = 23, Menpai = "華山", Kongfu = "葵花寶典", Level = 7},
                new MartialArtsMaster() {Id = 7, Name = "嶽不群", Age = 50, Menpai = "華山", Kongfu = "葵花寶典", Level = 8},
                new MartialArtsMaster() {Id = 8, Name = "令狐沖", Age = 23, Menpai = "華山", Kongfu = "獨孤九劍", Level = 10},
                new MartialArtsMaster() {Id = 9, Name = "梅超風", Age = 23, Menpai = "桃花島", Kongfu = "九陰真經", Level = 8},
                new MartialArtsMaster() {Id = 10, Name = "黃藥師", Age = 23, Menpai = "梅花島", Kongfu = "彈指神通", Level = 10},
                new MartialArtsMaster() {Id = 11, Name = "風清揚", Age = 23, Menpai = "華山", Kongfu = "獨孤九劍", Level = 10}
            };
            //初始化武學
            var kongfuList = new List<Kongfu>()
            {
                new Kongfu() {Id = 1, Name = "打狗棒法", Power = 90},
                new Kongfu() {Id = 2, Name = "降龍十八掌", Power = 95},
                new Kongfu() {Id = 3, Name = "葵花寶典", Power = 100},
                new Kongfu() {Id = 4, Name = "獨孤九劍", Power = 100},
                new Kongfu() {Id = 5, Name = "九陰真經", Power = 100},
                new Kongfu() {Id = 6, Name = "彈指神通", Power = 100}
            };
}

上面定義了兩個類，武林高手和武學，現在我們查詢所有武學級別大於8且門派為丐幫的武林高手。

首先我們使用原始的方法來查詢：

{
    var res = new List<MartialArtsMaster>();
    foreach (var temp in masterList)
    {
        if (temp.Level > 8 && temp.Menpai == "丐幫")
    {
    res.Add(temp);
}

1 使用LINQ查詢，表示式寫法

            //1,使用LINQ做查詢( 表示式寫法)
            var res = from m in masterList
                      //from後面設定查詢的集合
                      where m.Level > 8 && m.Menpai == "丐幫"
                      //where後面跟上查詢的條件
                      select m;//表示m的結果結合返回

使用LINQ查詢，擴充套件方法的寫法。其中Where是過濾操作符，根據返回bool值的Func委託引數過濾元素

        //過濾方法
        static bool Test1(MartialArtsMaster master)
        {
            if (master.Level > 8) return true;
            return false;
        }

        //2,擴充套件方法的寫法
        //var res = masterList.Where(Test1);
        var res = masterList.Where(m => m.Level > 8 && m.Menpai == "丐幫");

查詢結果：

LINQ也可以同時查多個表，下面我們取得所學功夫的殺傷力大於90 的武林高手。

2 LINQ 聯合查詢

            var res = from m in masterList
                      from k in kongfuList
                      where m.Kongfu == k.Name && k.Power > 90
                      select m;

LINQ 聯合查詢，擴充套件方法用法

            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);

查詢結果：

3 對查詢結果做排序 orderby (descending) 

優先等級排序，然後年齡排序

            var res = from m in masterList
                      where m.Level > 8 && m.Menpai == "丐幫" 
                      //orderby m.Age descending  // 預設從小到大，加上descending從大到小
                      orderby m.Level, m.Age //按照多個欄位進行排序，如果欄位的屬性相同，就按照第二個屬性排序
                      select m;//表示m的結果結合返回

排序擴充套件用法

var res = masterList.Where(m => m.Level > 8 && m.Menpai == "丐幫").OrderBy(m => m.Age);

如果排序判斷條件有多個，後面的排序要用ThenBy

var res = masterList.Where(m => m.Level > 8).OrderBy(m => m.Level).ThenBy(m => m.Age);

查詢結果：

4 Join on集合聯合查詢

            var res = from m in masterList
                      // join...in... 表示要連線的表，on後面為連線條件，等於要用equals，不能用==
                      join k in kongfuList on m.Kongfu equals k.Name
                      where k.Power > 90
                      select new { master = m, kongfu = k };

查詢結果：

 5 分組查詢 into groups 

把武林高手按照所學功夫分類，看一下那個功夫修煉的人數最多

            var res = from k in kongfuList
                      join m in masterList on k.Name equals m.Kongfu
                      into groups   //分組
                      orderby groups.Count()  // 這個可以獲得數量
                      select new { kongfu = k, count = groups.Count() };

查詢結果：

6 按照自身欄位分組 group by 

            var res = from m in masterList
                      group m by m.Kongfu
                          into g
                          select new { count = g.Count(), key = g.Key };//g.Key Key表示是按照那個屬性分的組

結果：

7 量詞操作符 any all   判斷集合中是否滿足某個條件 

any判斷集合中是否有一個滿足，all判斷集合中是否全部滿足

            bool res = masterList.Any(m => m.Menpai == "長留");
            Console.WriteLine(res);
            res = masterList.All(m => m.Menpai == "丐幫");
            Console.WriteLine(res);

結果：