博客

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApplication6
{
    class Program
    {
        static void Main(string[] args)
        {
            List<StudentNameModel> names = new List<StudentNameModel>();
            List<StudentAddressModel> addresses = new List<StudentAddressModel>();
            names.Add(new StudentNameModel
            {
                StudentID = "1",
                StudentName = "张三"
            });
            names.Add(new StudentNameModel
            {
                StudentID = "2",
                StudentName = "李四"
            });
            names.Add(new StudentNameModel
            {
                StudentID = "3",
                StudentName = "王五"
            });
            addresses.Add(new StudentAddressModel
            {
                StudentID = "1",
                StudentAddress = "浙江"
            });
            addresses.Add(new StudentAddressModel
            {
                StudentID = "2",
                StudentAddress = "江苏"
            });
            #region inner join
            var innerquery = from n in names
                             join a in addresses on n.StudentID equals a.StudentID
                             select new
                             {
                                 n.StudentID,
                                 n.StudentName,
                                 a.StudentAddress
                             };
            var innerrsult = innerquery.ToList();
            #endregion
            #region left join
            var leftquery = from n in names
                            join a in addresses on n.StudentID equals a.StudentID into na
                            from a in na.DefaultIfEmpty()
                            select new
                            {
                                n.StudentID,
                                n.StudentName,
                                StudentAddress = a == null ? "" : a.StudentAddress//判断地址是否存在
                            };
            var leftreuslt = leftquery.ToList();
            #endregion
        }
    }
    class StudentNameModel
    {
        public string StudentID { get; set; }
        public string StudentName { get; set; }
    }
    class StudentAddressModel
    {
        public string StudentID { get; set; }
        public string StudentAddress { get; set; }
    }
}

另一种写法

using System;
using System.Collections.Generic;
using System.Linq;

namespace ConsoleApp3
{
    class Program
    {
        static void Main(string[] args)
        {
            List<StudentDemo> student_list = GetStudentList();
            List<ClassDemo> class_list = GetClassList();
            List<StudentNew> students = student_list.Where(a => a.class_id == 1 || a.class_id == 2)
                .Join(class_list, s => s.class_id, c => c.class_id, (s, c) => new StudentNew
                {
                    student_id = s.student_id,
                    student_name = s.student_name,
                    class_id = s.class_id,
                    class_name = c.class_name
                }).ToList();
        }
        private static List<ClassDemo> GetClassList()
        {
            List<ClassDemo> list = new List<ClassDemo>();
            list.Add(new ClassDemo
            {
                class_id = 1,
                class_name = "班级1"
            });
            list.Add(new ClassDemo
            {
                class_id = 2,
                class_name = "班级2"
            });
            list.Add(new ClassDemo
            {
                class_id = 3,
                class_name = "班级3"
            });
            return list;
        }
        private static List<StudentDemo> GetStudentList()
        {
            List<StudentDemo> list = new List<StudentDemo>();
            list.Add(new StudentDemo
            {
                student_id = 1,
                student_name = "学生1",
                class_id = 1
            });
            list.Add(new StudentDemo
            {
                student_id = 2,
                student_name = "学生2",
                class_id = 1
            });
            list.Add(new StudentDemo
            {
                student_id = 3,
                student_name = "学生3",
                class_id = 2
            });
            list.Add(new StudentDemo
            {
                student_id = 4,
                student_name = "学生4",
                class_id = 2
            });
            list.Add(new StudentDemo
            {
                student_id = 5,
                student_name = "学生5",
                class_id = 3
            });
            return list;
        }
    }
    public class StudentDemo
    {
        public int student_id { get; set; }
        public string student_name { get; set; }
        public int class_id { get; set; }
    }
    public class ClassDemo
    {
        public int class_id { get; set; }
        public string class_name { get; set; }
    }
    public class StudentNew : StudentDemo
    {
        public string class_name { get; set; }
    }
}

lambda获取结果指向对象

var leftquery = from n in names
                            join a in addresses on n.StudentID equals a.StudentID into na
                            from a in na.DefaultIfEmpty()
                            select new { n, a };
            var leftreuslt = leftquery.ToList();

单个连接中的多个字段上进行LINQ连接

var query = from n in templist
            join d in deptList on n.DeptCode equals d.Code
            join p in positionList on new { JobTitleCode = n.JobTitleCode, DeptId = d.DeptId } 
            equals new { JobTitleCode = p.Code, DeptId = p.DeptId }
            join u in userList on n.LoginName equals u.LoginName
            select new SyncUserPositionDto
            {
                UserId = u.UserId,
                PositionId = p.PositionId,
                CreateTime = n.CreateTime,
                UpdateTime = n.UpdateTime,
            };
List<SyncUserPositionDto> list = query.ToList();