Linq 聚合函数

var WReserve = (from m in _db.W_RESERVE select m).ToList().LastOrDefault();

必须ToList() 之后才能 last first 函数，否则报错。这样会不会从数据库 取全部数据 然后再 last()；未测试。

 

using System;using System.Data;using System.Configuration;using System.Linq;using System.Web;using System.Web.Security;using System.Web.UI;using System.Text;using System.Web.UI.WebControls;using System.Web.UI.WebControls.WebParts;using System.Web.UI.HtmlControls;using System.Collections.Generic;using System.Collections;

public partial class _Default : System.Web.UI.Page{    protected void Page_Load(object sender, EventArgs e)    {        #region 用Count()得出查询结果的个数        Response.Write("<hr>用Count()得出查询结果的个数<br>");        var langCount = (from s in GetStudents()                           select s.Language)                           .Distinct()                           .Count();        Response.Write(string.Format("<div class='result'>{0}</div>", langCount));        #endregion

        #region 用Count()的Lambda表达式满足条件的元素个数        Response.Write("<hr>用Count()得出查询结果的个数<br>");        var ageCoun = (from s in GetStudents()                          select s.Age)                          .Count(a=>a>22);        Response.Write(string.Format("<div class='result'>{0}</div>", ageCoun));        #endregion

        #region 用Sum()将查询结果相加        Response.Write("<hr>用Sum()将查询结果相加<br>");        var ageSum = (from s in GetStudents()                      select s.Age)                       .Sum();        //var ageSum = GetStudents().Sum(s=>s.Age);        Response.Write(string.Format("<div class='result'>{0}</div>", ageSum));        #endregion

        #region 用Min()得出查询结果的最小值        Response.Write("<hr>用Min()得出查询结果的最小值<br>");        var ageMin = (from s in GetStudents()                      select s.Age)                       .Min();        //var ageMin = GetStudents().Min(s => s.Age);        Response.Write(string.Format("<div class='result'>{0}</div>", ageMin));        #endregion

        #region 用Max()得出查询结果的最大值        Response.Write("<hr>用Max()得出查询结果的最大值<br>");        var ageMax = (from s in GetStudents()                      select s.Age)                       .Max();        //var ageMax = GetStudents().Max(s => s.Age);        Response.Write(string.Format("<div class='result'>{0}</div>", ageMax));        #endregion

        #region 用Average()得出查询结果的平均值        Response.Write("<hr>用Average()得出查询结果的平均值<br>");        var ageAverage = (from s in GetStudents()                      select s.Age)                      .Average();        //var ageAverage = GetStudents().Average(s => s.Age);        Response.Write(string.Format("<div class='result'>{0}</div>", ageAverage));        #endregion    }

    #region 构造一个学生集合体    private List<Student> GetStudents()    {        List<Student> students = new List<Student> {             new Student{ Name="YOUNG", Age=25, Language="Chinese"},             new Student{ Name="JESSIE", Age=21, Language="Scotland"},             new Student{ Name="KELLY", Age=18, Language="English"},             new Student{ Name="JUNE", Age=20, Language="English"},             new Student{ Name="ADRIAN", Age=22, Language="Italy"},             new Student{ Name="BRUCE", Age=17, Language="Scotland"},             new Student{ Name="BRANT", Age=30, Language="Germany"},             new Student{ Name="BEN", Age=25, Language="Chinese"}        };        return students;    }    #endregion}

#region 学生类class Student{    public int Age { get; set; }    public string Name { get; set; }    public string Language { get; set; }}#endregion

转载于:https://www.cnblogs.com/90nice/p/3672291.html

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