Linq To sql入门练习 Lambda表达式基础

2019年3月23日 151次阅读来源: 木头园—OOIP

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace 练习LinQ入门
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        private mydbDataContext context = new mydbDataContext();

        private void Form1_Load(object sender, EventArgs e)
        {
            FillInfo();
            cbx_sex.SelectedIndex = 0;
        }

        private void FillInfo()
        {
            //法一：
            //List<Info> list = context.Info.ToList();
            //dataGridView1.DataSource = list;
            //法二：
            //var query = from p in context.Info select p;
            //dataGridView1.DataSource = query;
            //法三：
            var query = context.Info;
            dataGridView1.DataSource = query;
        }

        private void btn_ADD_Click(object sender, EventArgs e)
        {
            Info data = new Info();
            data.Code = txt_code.Text;
            data.Name = txt_name.Text;
            data.Nation = txt_nation.Text;
            data.Sex = Convert.ToBoolean(txt_sex.Text);
            data.Birthday = Convert.ToDateTime(txt_b.Text);

            context.Info.InsertOnSubmit(data);
            context.SubmitChanges();//提交
            FillInfo();
        }

        private void button2_Click(object sender, EventArgs e)
        {
            var query = from p in context.Info where p.Code == txt_code.Text select p;
            if (query.Count() > 0)
            {
                // query.Single();//第一条
                Info data = query.First();
                context.Info.DeleteOnSubmit(data);
                context.SubmitChanges();
            }
            FillInfo();

            //Form p in context.Info where 
            //   Info data = context.Info.Where( => );
        }

        private void button1_Click(object sender, EventArgs e)
        {
            var a = from p in context.Info where p.Code == txt_code.Text select p;
            if (a.Count() > 0)
            {
                Info data = a.First();
                data.Name = txt_name.Text;
                data.Sex = Convert.ToBoolean(txt_sex.Text);
                data.Nation = txt_nation.Text;
                data.Birthday = Convert.ToDateTime(txt_b.Text);
                context.SubmitChanges();
            }
            FillInfo();
        }

        private void btn_select_Click(object sender, EventArgs e)
        {

            #region meiyong
            //// e.Equals();
            //Type t = e.GetType();
            ////t.GetProperties();
            ////t.GetMethods();
            //Type s = sender.GetType(); 
            #endregion
            Button btn = (Button)sender;
            if (btn.Name == "btn_select")
            {
                if (tbx_code2.Text != "")
                {
                    //单条件等值查询
                    //var query = from p in context.Info where p.Code == tbx_code2.Text.Trim() select p;

                    var query = context.Info.Where(p => p.Code == tbx_code2.Text.Trim());//扩展方法 Lambda表达式
                    dataGridView1.DataSource = query;
                }
            }
            else if (btn.Name == "btn_select2")
            {
                if (tbx_code2.Text != "")
                {
                    //单条件不等值查询
                    //var query = from p in context.Info where p.Code != tbx_code2.Text.Trim() select p;
                    //>=  <=  同样可以用于筛选
                    var query = context.Info.Where(p => p.Code != tbx_code2.Text.Trim());
                    dataGridView1.DataSource = query;
                }
            }
            else if (btn.Name == "btn_duo")
            {
                //多条件查询
                var query = from p in context.Info where p.Sex == Convert.ToBoolean(cbx_sex.Text.Trim()) && p.Birthday.Value.Year > Convert.ToInt32(tbx_b_duo.Text.Trim()) select p;
                //或者
                //var query1 = context.Info.Where(p => p.Sex == Convert.ToBoolean(cbx_sex.Text.Trim()) && p.Birthday.Value.Year > Convert.ToInt32(tbx_b_duo.Text.Trim()));
                //var a=context.Info.Where (p=>p.Sex==Convert.ToBoolean(cbx_sex.Text.Trim())).Where(p=>p.Birthday.Value.Year> Convert.ToInt32(tbx_b_duo.Text.Trim()));//链式表达式 （&& 逻辑与可以用） 
                dataGridView1.DataSource = query;
            }
            else if (btn.Name == "btn_select_mohu")
            {
                //var query = from p in context.Info where p.Name.Contains(tbx_xing.Text.Trim()) select p;//相当于like'%某'
                //var query = from p in context.Info where p.Name.StartsWith(tbx_xing.Text.Trim()) select p;//以什么开头
                //var query = from p in context.Info where p.Name.EndsWith(tbx_xing.Text.Trim()) select p;//以什么结尾
                //var query = from p in context.Info where p.Name.Substring(1, tbx_xing.TextLength) == tbx_xing.Text.Trim() select p;//截取第一位往后
                var query = context.Info.Where(p => p.Name.StartsWith(tbx_xing.Text.Trim()));

                dataGridView1.DataSource = query;
            }
            else if (btn.Name == "btn_shua")
            {
                FillInfo();
            }
            else if (btn.Name == "btn_jiao")
            {
                //交集相当于逻辑与 取公共部分
                var p1 = from p in context.Info where p.Sex == true select p;
                var p2 = context.Info.Where(p => p.Birthday.Value.Year > 1985);
                var query = p1.Intersect(p2);
                dataGridView1.DataSource = query;
            }
            else if (btn.Name == "btn_bing")
            {
                //并集相当于逻辑或
                var p1 = from p in context.Info where p.Sex == true select p;
                var p2 = context.Info.Where(p => p.Birthday.Value.Year > 1985);
                var query = p1.Union(p2);
                dataGridView1.DataSource = query;
            }
            else if (btn.Name == "btn_bu")
            {
                //差集 从一个集合减去另外一个集合 看谁减谁
                var p1 = from p in context.Info where p.Sex == true select p;
                var p2 = context.Info.Where(p => p.Birthday.Value.Year > 1985);
                var query = p1.Except(p2);
                dataGridView1.DataSource = query;
            }

        }

        private void btn_jige_Click(object sender, EventArgs e)
        {
           Button btn =(Button)sender;
           if (btn.Name=="btn_jige")
           {
               var query = context.Info.Where(p => p.Birthday.Value.Year > 1985);
               this.Text = query.Count().ToString();//个数
           }
           else if (btn.Name=="btn_sum")
           {
               var query = context.Info.Where(p => p.Birthday.Value.Year > 1985);
               this.Text = query.Sum(p=>DateTime.Now.Year-p.Birthday.Value.Year).ToString();//求和
           }
           else if (btn.Name == "btn_avg")
           {
               var query = context.Info.Where(p => p.Birthday.Value.Year > 1985);
               this.Text = query.Average(p => DateTime.Now.Year - p.Birthday.Value.Year).ToString();//求平均值
           }

        }

        private void button5_Click(object sender, EventArgs e)
        {
            var query = context.Info.Where(p => p.Birthday.Value.Year > 1985);
            List<Info> list = query.ToList();//转集合
            dataGridView1.DataSource = list;
        }

        private void button7_Click(object sender, EventArgs e)
        {
            var query = context.Info.Where(p => p.Birthday.Value.Year > 1985);
            Info[] list = query.ToArray();//转数组
            dataGridView1.DataSource = list;
        }

        private void button6_Click(object sender, EventArgs e)
        {
            var query = context.Info.Where(p => p.Birthday.Value.Year > 1985);
            //Info data = query.Single();//单一
            Info data = query.First();
            dataGridView1.DataSource = data;
        }

        private void button3_Click(object sender, EventArgs e)
        {
            //每页多少条pagesize，要找第几页pageno
            var query=context.Info.Skip(1*1).Take(2);//skip跳过多少行，take取多少行
            dataGridView1.DataSource=query;
        }

    }
}

练习源码：http://pan.baidu.com/s/1c0wJ8i8

LinQ to sql 入门小教程：http://pan.baidu.com/s/1dD2NkcD

1.查询所有的

var query = from p in _Context.Info select p;
var query = _Context.Info;

2.单条件查询

等值查
var query = from p in _Context.Info where p.Code == “p002” select p;
注意：双等号，双引号。—C#语法。
var query = _Context.Info.Where(p => p.Code == “p002” ); //Lambda表达式
不等值查
            //var query = from p in _Context.Info where p.Code != “p002” select p;
            //var query = _Context.Info.Where(p => p.Code != “p002”); //Lambda表达式
            //var query = from p in _Context.Info where p.Birthday.Value.Year > 1985 select p;
            var query = _Context.Info.Where(p=>p.Birthday.Value.Year < 1985);

3.多条件查询

            //var query = from p in _Context.Info where p.Sex == true && p.Birthday.Value.Year > 1985 select p;
            //var query = _Context.Info.Where(p=>p.Sex==true && p.Birthday.Value.Year>1985);
            var query = _Context.Info.Where(p => p.Sex == true).Where(p=>p.Birthday.Value.Year>1985) ; //多条件逻辑与可以用链式表达式写

4.模糊查询

            //var query = from p in _Context.Info where p.Name.Contains(“张”) select p; //相当于like ‘%张%’
            //var query = from p in _Context.Info where p.Name.StartsWith(“张”) select p; // like ‘张%’
            //var query = from p in _Context.Info where p.Name.EndsWith(“张”) select p; //like ‘%张’
            //var query = from p in _Context.Info where p.Name.Substring(1,1) == “张” select p;// like ‘_张%’

var query = _Context.Info.Where(p => p.Name.StartsWith(“张”));

5.集合操作

var q1 = from p in _Context.Info where p.Sex== true select p;
var q2 = _Context.Info.Where(p => p.Birthday.Value.Year > 1985);

交
var query = q1.Intersect(q2);
并
var query = q1.Union(q2);
差
var query = q2.Except(q1);

6.统计函数：

            var query = _Context.Info.Where(p => p.Birthday.Value.Year > 1985);
            //this.Text = query.Count().ToString();
            //this.Text = query.Sum(p=>DateTime.Now.Year – p.Birthday.Value.Year).ToString();
            //this.Text = query.Average(p => DateTime.Now.Year – p.Birthday.Value.Year).ToString();
            //this.Text = query.Max(p => DateTime.Now.Year – p.Birthday.Value.Year).ToString();
            this.Text = query.Min(p => DateTime.Now.Year – p.Birthday.Value.Year).ToString();

7.转换操作

ToList(),ToArray(),First()/Single()

8.分页

var query = _Context.Info.Skip(2*2).Take(2);

这里主要是将数据库中的常用操作用LAMBDA表达式重新表示了下，用法不多，但相对较常用，等有时间了还会扩展，并将查询语句及LINQ到时也一并重新整理下：

1.select语句：books.Select(p=>new { p.Title, p.UnitPrice, p.Author});//需用匿名方式

2.where语句：books.Where(p=>p.UnitPrice==100&&p.Title=”ABC”);

补充：

像数据库中的LIKE ‘%c++%’，LAMBDA中用p.Title.Contains(“c++”)表示；

像数据库中的LIKE ‘c%’，LAMBDA中用p.Title.StartWith(“c”)表示；

像数据库中的LIKE ‘%c’，LAMBDA中用p.Title.EndsWith(“c”)表示；

Where的另一种表现形式：

books.Where(p=>{

    var ret = p.UnitPrice>30&&p.Title.Contains(“c++”);

    return ret;

});

3.排序语句：

像数据库中order by 升序：

通过 “对象.OrderBy(p=>p.UnitPrice)”实现

像数据库中order by 降序：

通过 “对象.OrderByDescending(p=>p.UnitPrice)”实现

像数据库中order by UnitPrice desc，Title asc：

通过 ”对象.OrderByDescending(p=>p.UnitPrice).ThenBy(p=>p.Title)”

反过来则是： ”对象.OrderBy(p=>p.UnitPrice).ThenByDescending(p=>p.Title)”

 

4.组函数：

  var max = books.Where(p => p.CategoryId == 1001).Max(p => p.UnitPrice);

        var min = books.Min(p => p.UnitPrice);

        var count = books.Count( );

        var avg = books.Average(p => p.UnitPrice);

        var sum = books.Sum(p => p.UnitPrice);

注意，上面这些获得的东西，不是对象，是单个值

 

5. GROUP BY函数

// select categoryid,max(unitpirce) from books group by categoryid having max(unitprice)>50

        var list6 = books.GroupBy(p => p.CategoryId).Where(p=>p.Max(q=>q.UnitPrice)>50);

        foreach (var item in list6)

        { 

            Response.Write(string.Format("类别编号:{0},最高价{1} ", 
item.Key,item.Max(p=>p.UnitPrice)));

        }

 
 

6. TOP函数

//取一个范围 如3,5

var list7 = books.Skip(2).Take(3).Select(p => new { p.Title, p.CategoryId,  p.UnitPrice });

// select top 5 

var list7 = books.Take(5).OrderByDescending(p => p.UnitPrice)

 .Select(p => new { p.CategoryId, p.UnitPrice, p.Title, p.Author });

 

7.union 函数

books.Where(p => p.CategoryId == 1001).Select(p => new { p.CategoryId, p.UnitPrice, p.Title, p.Author }).Union(books.Where(p => p.CategoryId == 1002).Select(p => new { p.CategoryId, p.UnitPrice, p.Title, p.Author }));

这里的Select子句中的列需对应，跟数据库中是一样的

 

8.Join方法，用于实现数据库中双表连接查询

//select a.title,a.unitprice,a.categoryid,b.id,b.name from books a,category b 

//where a.categoryid=b.id and b.name=‘数据库’

books.Join(cates.Where(m => m.Name == "数据库"),p => p.CategoryId, q => q.ID, (a, b) => new { a.Title, a.UnitPrice, a.CategoryId, b.ID, b.Name });

说明：

Join()方法的调用对象类似于在SQL语句中第一张表的表名

而Join()方法的第一个形参是第二张表表名的Where条件

Join()方法的第二和第三个参数分别表示第一张表与第二张表的关联字段

Join()方法的第四个参数表示从两表中需要获取的字段，(a, b)分别表示第一张表和第二张表

http://blog.csdn.net/codefighting/article/details/7048966

.AsEnumerable()是延迟执行的，实际上什么都没有发生，当真正使用对象的时候（例如调用：First, Single, ToList….的时候）才执行。
下面就是.AsEnumerable()与相对应的.AsQueryable()的区别：
AsEnumerable将一个序列向上转换为一个IEnumerable, 强制将Enumerable类下面的查询操作符绑定到后续的子查询当中。
AsQueryable将一个序列向下转换为一个IQueryable, 它生成了一个本地查询的IQueryable包装。

.AsEnumerable()延迟执行，不会立即执行。当你调用.AsEnumerable()的时候，实际上什么都没有发生。
.ToList()立即执行
当你需要操作结果的时候，用.ToList()，否则，如果仅仅是用来查询不需要进一步使用结果集，并可以延迟执行，就用.AsEnumerable()/IEnumerable /IQueryable
.AsEnumerable()虽然延迟执行，但还是访问数据库，而.ToList()直接取得结果放在内存中。比如我们需要显示两个部门的员工时，部门可以先取出放置在List中，然后再依次取出各个部门的员工，这时访问的效率要高一些，因为不需要每次都访问数据库去取出部门。
IQueryable实现了IEnumberable接口。但IEnumerable<T> 换成IQueryable<T>后速度提高很多。原因：
IQueryable接口与IEnumberable接口的区别： IEnumerable<T> 泛型类在调用自己的SKip 和 Take 等扩展方法之前数据就已经加载在本地内存里了，而IQueryable<T> 是将Skip ,take 这些方法表达式翻译成T-SQL语句之后再向SQL服务器发送命令，它并不是把所有数据都加载到内存里来才进行条件过滤。
IEnumerable跑的是Linq to Object，强制从数据库中读取所有数据到内存先。

    原文作者：木头园—OOIP
    原文地址: https://www.cnblogs.com/ooip/p/4715202.html
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