数据准备：

if exists (select * from sysobjects where id = OBJECT_ID('[classinfo]') and OBJECTPROPERTY(id, 'IsUserTable') = 1) 
DROP TABLE [classinfo]

CREATE TABLE [classinfo] (
[id] [bigint]  NOT NULL,
[classID] [bigint]  NOT NULL,
[className] [nvarchar]  (50) NOT NULL,
[stat] [varchar]  (2) NOT NULL DEFAULT (1),
[autoid] [bigint]  IDENTITY (1, 1)  NOT NULL)

ALTER TABLE [classinfo] WITH NOCHECK ADD  CONSTRAINT [PK_classinfo] PRIMARY KEY  NONCLUSTERED ( [id] )
SET IDENTITY_INSERT [classinfo] ON

INSERT [classinfo] ([id],[classID],[className],[stat],[autoid]) VALUES ( 1,1,N'计算机一班',N'1',1)
INSERT [classinfo] ([id],[classID],[className],[stat],[autoid]) VALUES ( 2,2,N'计算机二班',N'1',2)
INSERT [classinfo] ([id],[classID],[className],[stat],[autoid]) VALUES ( 4,3,N'计算机三班',N'0',3)
INSERT [classinfo] ([id],[classID],[className],[stat],[autoid]) VALUES ( 5,4,N'计算机四班',N'1',4)
INSERT [classinfo] ([id],[classID],[className],[stat],[autoid]) VALUES ( 6,5,N'计算机五班',N'1',6)

SET IDENTITY_INSERT [classinfo] OFF


if exists (select * from sysobjects where id = OBJECT_ID('[stuinfo]') and OBJECTPROPERTY(id, 'IsUserTable') = 1) 
DROP TABLE [stuinfo]

CREATE TABLE [stuinfo] (
[id] [bigint]  IDENTITY (1, 1)  NOT NULL,
[username] [nvarchar]  (50) NULL,
[userpwd] [nvarchar]  (50) NULL,
[classID] [bigint]  NULL,
[score] [numeric]  (10,2) NULL,
[age] [int]  NULL,
[CreateTime] [datetime]  NULL DEFAULT (getdate()))

alter TABLE [stuinfo] WITH NOCHECK ADD  CONSTRAINT [PK_stuinfo] PRIMARY KEY  NONCLUSTERED ( [id] )
SET IDENTITY_INSERT [stuinfo] ON

INSERT [stuinfo] ([id],[username],[userpwd],[classID],[score],[age],[CreateTime]) VALUES ( 1,N'001',N'0004',1,123.22,25,N'2012/8/24 10:58:10')
INSERT [stuinfo] ([id],[username],[userpwd],[classID],[score],[age],[CreateTime]) VALUES ( 2,N'002',N'154',2,888.00,21,N'2012/8/24 10:58:10')
INSERT [stuinfo] ([id],[username],[userpwd],[classID],[score],[age],[CreateTime]) VALUES ( 3,N'003',N'555',1,888.00,16,N'2012/8/24 10:58:10')
INSERT [stuinfo] ([id],[username],[userpwd],[classID],[score],[age],[CreateTime]) VALUES ( 4,N'004',N'644',2,85.60,18,N'2012/8/24 10:58:10')
INSERT [stuinfo] ([id],[username],[userpwd],[classID],[score],[age],[CreateTime]) VALUES ( 5,N'pkm',N'123',3,46.00,19,N'2012/8/24 10:58:10')
INSERT [stuinfo] ([id],[username],[userpwd],[classID],[score],[age],[CreateTime]) VALUES ( 6,N'pkm001',N'123',3,45.56,19,N'2012/8/24 10:58:10')
INSERT [stuinfo] ([id],[username],[classID],[CreateTime]) VALUES ( 7,N'2012pkm1',1,N'2012/8/24 10:58:10')

SET IDENTITY_INSERT [stuinfo] OFF

建立3.5以上版本的项目，添加 ADO.NET实体数据模型，连接数据库。（文件后缀为：edmx）

            using (var edm = new testEntities())
            {
                #region linq to entities 内容练习
               // /*
                ObjectQuery<stuinfo> stu = edm.stuinfo;
                IQueryable<stuinfo> stu1 = from s in stu
                                           select s;
                //Assert.Greater(stu1.Count(), 0);

                Console.WriteLine(stu.ToTraceString());//跟踪输出语句

                //where条件选择
                stu1 = from s in stu
                       where s.classID == 3 //选择班级id为3的记录
                       select s;
                //stu1 = stu.Where(s => s.classID==3);
                Console.WriteLine(stu1.Count());
                foreach (var s in stu1)
                {
                    Console.WriteLine("name:" + s.username);//输出用户姓名
                }

                //orderby排序
                stu1 = (from s in stu
                        orderby s.id descending //选择班级id为3的记录
                        select s).Skip(3).Take(3);//跳过3条记录，再取3条记录
                //stu1 = stu.OrderByDescending(s => s.id).Skip(3).Take(3);
                Console.WriteLine(stu1.Count());
                foreach (var s in stu1)
                {
                    Console.WriteLine("name:" + s.username);//输出用户姓名
                }

                //聚合函数
                var maxid = edm.stuinfo.Max(p => p.id);//查询字段中最大的值
                Console.WriteLine(maxid);
                // */
                var query = from s in edm.stuinfo
                            join c in edm.classinfo
                            on s.classID equals c.classID //两表关联条件
                            where s.username == "pkm" //查询条件
                            select new
                            {
                                //格式：列名=列值
                                id = s.id,
                                username = s.username,
                                userpwd = s.userpwd,
                                className = c.className,
                                score = s.score >= 60 ? s.score : 0 //少于60分成绩为0
                            };
                foreach (var sc in query)
                {
                    Console.WriteLine("{0} {1} {2} {3}", sc.className, sc.username, sc.userpwd, sc.score);
                }

                var stu11 = from s in edm.stuinfo
                            where s.CreateTime >= new DateTime(2012, 8, 24, 10, 58, 10)// && s.username == "pkm"//根据时间和姓名查询
                            select s;
                Console.WriteLine(stu11.Count());//数量，参数可以加条件  s => s.username=="pkm"
                foreach (var s in stu11)
                {
                    Console.WriteLine("name:" + s.username);//输出用户姓名
                }
                //stuinfo stu01 = stu1.First();//返回集合中第一个（强类型对象）

                var cls = (from c in edm.classinfo
                           select c.className).Distinct();//distinct
                foreach (var c in cls)
                {
                    Console.WriteLine("classname:" + c);//输出用户姓名
                }
                var count = edm.stuinfo.Select(s => s.score).Sum();
               // count = edm.stuinfo.Sum(s => s.score); //用法2(Min()、Max() 也是一样的用法)
                Console.WriteLine("分数之和为：{0}",count);

                #endregion
            }