Linq、lambda 多表连接查询join

在查询语言中，通常需要使用联接操作。在 LINQ 中，可以通过 join 子句实现联接操作。join 子句可以将来自不同源序列，并且在对象模型中没有直接关系(数据库表之间没有关系)的元素相关联，唯一的要求是每个源中的元素需要共享某个可以进行比较，以判断是否相等的值。

在 LINQ 中，join 子句可以实现 3 种类型的联接分别是内部联接、分组联接和左外部联接。 

1、内部连接(相对于sql：join | inner join)

格式：join element in dataSource on exp1 equals exp2 

            Console.WriteLine("\r\n---------------内部连接------------------");
            int[] intAry1 = { 5, 15, 25, 30, 33, 40 };//创建整数数组 intAry1 作为数据源
            int[] intAry2 = { 10, 20, 30, 50, 60, 70, 80 };//创建整数数组 intAry2 作为数据源
            //查询 query1 使用 join 子句从两个数据源获取数据
            //演示内部联接的使用
            var query1 =
            from val1 in intAry1
            join val2 in intAry2 on val1 % 5 equals val2 % 15
            select new { VAL1 = val1, VAL2 = val2 };
            foreach (var item in query1)
            {
                Console.WriteLine(item);
            }

结果：

2、分组连接

格式： join element in dataSource on exp1 equals exp2 into grpName 

其中，into 关键字表示将这些数据分组并保存到 grpName 中，grpName 是保存一组数据的集合。（感觉和sql不同，sql查询的结果是平面矩形的，而linq则是平面树形的，意思是像对象的元素也是个对象）  

 Console.WriteLine("-----------分组连接--------------");
            int[] intAry1 = { 5, 15, 25, 30, 33, 40 };//创建整数数组 intAry1 作为数据源
            int[] intAry2 = { 10, 20, 30, 50, 60, 70, 80 };//创建整数数组 intAry2 作为数据源
            //查询 query1 使用 join 子句从两个数据源获取数据
            //演示分组联接的使用
            var query1 =
            from val1 in intAry1
            join val2 in intAry2 on val1 % 5 equals val2 % 15 into val2Grp
            select new { VAL1 = val1, VAL2GRP = val2Grp };
            foreach (var item in query1)
            {
                Console.WriteLine(item.VAL1 + "--" + string.Join(",", item.VAL2GRP.ToArray()));
            }

结果：

 

3、左外部联接 (相对于sql：left join | left outer join)

第三种联接是左外部联接，它返回第一个集合中的所有元素，无论它是否在第二个集合中有相关元素。在 LINQ 中，通过对分组联接的结果调用 DefaultIfEmpty()方法来执行左外部联接。DefaultIfEmpty()方法从列表中获取指定元素。如果列表为空，则返回默认值。

  Console.WriteLine("-------------左外部链接----------------------");
            int[] intAry1 = { 5, 15, 23, 30, 33, 40 };//创建整数数组 intAry1 作为数据源
            int[] intAry2 = { 10, 20, 30, 50, 60, 70, 80 };//创建整数数组 intAry2 作为数据源
            //查询 query1 使用 join 子句从两个数据源获取数据
            //演示左联接的使用
            var query1 =
            from val1 in intAry1
            join val2 in intAry2 on val1 % 5 equals val2 % 15 into val2Grp
            from grp in val2Grp.DefaultIfEmpty()
            select new { VAL1 = val1, VAL2GRP = grp };
            foreach (var item in query1)
            {
                Console.WriteLine(item.VAL1 + "--" + item.VAL2GRP);
            }

结果：

------------------------------------------------------------------------------------------

查询方法Lambda示例（GroupJoin）

public static IEnumerable<TResult> GroupJoin<TOuter, TInner, TKey, TResult>(
    this IEnumerable<TOuter> outer,
    IEnumerable<TInner> inner,
    Func<TOuter, TKey> outerKeySelector,
    Func<TInner, TKey> innerKeySelector,
    Func<TOuter, IEnumerable<TInner>, TResult> resultSelector
)

重载

public static IEnumerable<TResult> GroupJoin<TOuter, TInner, TKey, TResult>(
    this IEnumerable<TOuter> outer,
    IEnumerable<TInner> inner,
    Func<TOuter, TKey> outerKeySelector,
    Func<TInner, TKey> innerKeySelector,
    Func<TOuter, IEnumerable<TInner>, TResult> resultSelector,
    IEqualityComparer<TKey> comparer
)

左链接查询组合：

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

namespace TestKzt
{
    #region
    /// <summary>
    /// 学生实体
    /// </summary>
    public class Student
    {
        public int StudentId { get; set; }
        public string StudentName { get; set; }
        public int Age { get; set; }
        public int StandardId { get; set; }//水平
        public int GradeId { get; set; }
    }
    /// <summary>
    /// 水平/等级
    /// </summary>
    public class Standard
    {
        public int StandardId { get; set; }
        public string StandardName { get; set; }//
    }
    public class GradeClass
    {
        public int GradeId { get; set; }
        public string GradeName { get; set; }
    }
    class Program
    {
        static void Main(string[] args)
        {
            #region 数据源
            IList<Student> studentList = new List<Student>()
            {
                new Student() {StudentId = 1, StudentName = "John", StandardId = 1,Age=2,GradeId=1},
                new Student() {StudentId = 2, StudentName = "Moin", StandardId = 1,Age=3,GradeId=1},
                new Student() {StudentId = 3, StudentName = "Bill", StandardId = 2,Age=5,GradeId=2},
                new Student() {StudentId = 4, StudentName = "Ram", StandardId = 2,Age=20},
                new Student() {StudentId = 5, StudentName = "Ron", StandardId = 5},
                 new Student() {StudentId = 10, StudentName = "张华", StandardId = 5},
                  new Student() {StudentId = 20, StudentName = "李明", StandardId = 5},
                  new Student() {StudentId = 30, StudentName = "王五", StandardId = 5}
            };
            IList<Standard> standardList = new List<Standard>()
            {
                new Standard() {StandardId = 1, StandardName = "优秀"},
                new Standard() {StandardId = 2, StandardName = "中等"},
                new Standard() {StandardId = 3, StandardName = "差生"}
            };
            IList<GradeClass> GradeClassList = new List<GradeClass>() { 
            new GradeClass() {GradeId=1,GradeName="一年级"},
            new GradeClass(){GradeId=2,GradeName="二年级"}
            };
            #endregion
            Console.WriteLine("----三个表linq内连接");
            var q0 = from a in studentList
                    join b in standardList on a.StudentId equals b.StandardId                 
                    join g in GradeClassList on a.GradeId equals g.GradeId
                     where a.StudentId != 2 && !a.StudentName.Contains("John")
                    select new
                    {
                        a.StudentId,
                        a.StudentName,
                        a.Age,
                        a.StandardId,
                        StandarFullName =b.StandardName,
                        a.GradeId,
                        GradeName =g.GradeName 
                    };
            foreach (var item in q0)
            {
                var str = string.Format("学生ID:{0},姓名:{1},年龄:{2},水平ID:{3},水平:{4},班级ID:{5},班级:{6}", item.StudentId, item.StudentName, item.Age, item.StandardId, item.StandarFullName, item.GradeId, item.GradeName);
                Console.WriteLine(str);
            }
            Console.WriteLine("----三个表linq 左连接");
            var q = from a in studentList
                    join b in standardList on a.StudentId equals b.StandardId into Lb_join
                    from lb in Lb_join.DefaultIfEmpty()
                    join g in GradeClassList on a.GradeId equals g.GradeId into Lg_join
                    from lg in Lg_join.DefaultIfEmpty()
                    select new
                    {
                        a.StudentId,
                        a.StudentName,
                        a.Age,
                        a.StandardId,
                        StandarFullName = lb != null ? lb.StandardName : "没有对应值",
                        a.GradeId,
                        GradeName = lg != null ? lg.GradeName : "没有班级"
                    };
            foreach (var item in q)
            {
                var str = string.Format("学生ID:{0},姓名:{1},年龄:{2},水平ID:{3},水平:{4},班级ID:{5},班级:{6}", item.StudentId, item.StudentName, item.Age, item.StandardId, item.StandarFullName, item.GradeId, item.GradeName);
                Console.WriteLine(str);
            }
            Console.WriteLine("\r\n----俩个表lambda 左连接");
            Console.WriteLine("\r\n----  方法一  ------\r\n");
            //方式 一
            //查询公式
            var groupJoin = standardList.GroupJoin(studentList,
                standard => standard.StandardId,
                student => student.StandardId,
                (standard, studentGroup) => new
                {
                    StandarFullName = standard.StandardName,
                    Students = studentGroup
                });//感觉和字典类型一样，一个key，对应一个velue, velue = IEnumerable<Student>

            //执行查询
            foreach (var item in groupJoin)
            {
                var str = item.StandarFullName;
                str += "【";
                foreach (var student in item.Students)
                {
                    str += student.StudentName + "-" + student.Age;
                }
                str += "】";
                Console.WriteLine(str);
            }
            Console.WriteLine("\r\n------方法二-------\r\n");
            //方式二
            var list = standardList.GroupJoin(studentList,
                x => x.StandardId,
                y => y.StandardId,
                (x, y) => y.DefaultIfEmpty(new Student()).Select(z => new { StandarFullName = x.StandardName, StudentName = z.StudentName, StandardId = z.StandardId, Age = z.Age })).SelectMany(x => x);
            foreach (var item in list)
            {
                var str = item.StandarFullName;
                str += item.StudentName + "-" + item.Age; ;
                Console.WriteLine(str);
            }
            Console.WriteLine("--------三表联合内连接查询---------");
            var list0 = studentList.Join(standardList, x => x.StandardId, y => y.StandardId, (x, y) => new {
               stu=x,
               Standard = y
            }).Where(x=>x.stu.StudentId!=2).Join(GradeClassList,m=>m.stu.GradeId,n=>n.GradeId,(m,n)=>new{
                m.stu.StudentId,
                m.stu.StudentName,
                m.stu.StandardId,
                m.Standard.StandardName,
                m.stu.GradeId,
                n.GradeName
            }).OrderByDescending(x=>x.StudentId);
            foreach (var item in list0)
            {
                StringBuilder sb = new StringBuilder();
                sb.Append("学生ID:" + item.StudentId);
                sb.Append(",姓名：" + item.StudentName);
                sb.Append(",水平ID：" + item.StandardId);
                sb.Append(",水平：" + item.StandardName);
                sb.Append(",班级：" + item.GradeName);
                Console.WriteLine(sb.ToString());
            }
            Console.WriteLine("--------三表联合左连接查询---------");
            //直接将列名提取
            //var list2 = studentList.GroupJoin(standardList, x => x.StandardId, y => y.StandardId, (x, y) => y.DefaultIfEmpty(new Standard()).Select(z => new
            //{
            //    x.StudentId,
            //    x.StudentName,
            //    x.StandardId,
            //    z.StandardName,
            //    x.GradeId
            //})).SelectMany(x => x).Where(x=>x.StudentId<20).GroupJoin(GradeClassList,m=>m.GradeId,n=>n.GradeId,(m,n)=>n.DefaultIfEmpty(new GradeClass()).Select(y=>new {
            //m.StudentId,
            //m.StudentName,
            //m.StandardId,
            //m.StandardName,
            //m.GradeId,
            //y.GradeName
            //})).SelectMany(x=>x);
            //将类对象作为子元素列
            var list2 = studentList.GroupJoin(standardList, x => x.StandardId, y => y.StandardId, (x, y) => y.DefaultIfEmpty(new Standard()).Select(z => new
            {
                stu=x,
                Standard=z
            })).SelectMany(x => x).Where(x => x.stu.StudentId < 20)
            .GroupJoin(GradeClassList, m => m.stu.GradeId, n => n.GradeId, (m, n) => n.DefaultIfEmpty(new GradeClass()).Select(y => new
            {
                m.stu.StudentId,
                m.stu.StudentName,
                m.stu.StandardId,
                m.Standard.StandardName,
                m.stu.GradeId,
                y.GradeName
            })).SelectMany(x => x);

            foreach (var item in list2)
            {
                StringBuilder sb = new StringBuilder();
                sb.Append("学生ID:"+item.StudentId);
                sb.Append(",姓名："+item.StudentName);
                sb.Append(",水平ID："+item.StandardId);
                sb.Append(",水平："+item.StandardName);
                sb.Append(",班级："+item.GradeName);
                Console.WriteLine(sb.ToString());
            }

            Console.WriteLine("------两表左连接-------");
            var list3 = studentList.GroupJoin(standardList, x => x.StandardId, y => y.StandardId, (x, y) => new { stu = x, stan = y })
                .Where(x=>x.stu.StudentId<30)
                .SelectMany(x => x.stan.DefaultIfEmpty(), (x, y) => new {
                StudentId=x.stu.StudentId,
                StudentName=x.stu.StudentName,
                StandardId=x.stu.StandardId,
                StandardName=y!=null?y.StandardName:"无值"
            });

            foreach (var item in list3)
            {
                StringBuilder sb = new StringBuilder();
                sb.Append("学生ID:" + item.StudentId);
                sb.Append(",姓名：" + item.StudentName);
                sb.Append(",水平ID：" + item.StandardId);
                sb.Append(",水平：" + item.StandardName);
                //sb.Append(",班级：" + item.GradeName);
                Console.WriteLine(sb.ToString());
            }
            Console.WriteLine("------三表左连接-------");
            var list4 = studentList.GroupJoin(standardList, x => x.StandardId, y => y.StandardId, (x, y) => new { stu = x, stan = y })
                .Where(x => x.stu.StudentId < 30)
                .SelectMany(x => x.stan.DefaultIfEmpty(), (x, y) => new { 
                stu=x.stu,
                StudentName = y != null ? y.StandardName : "无值"
                })
                .GroupJoin(GradeClassList, m => m.stu.GradeId, n => n.GradeId, (m, n) => new { stu = m.stu, StudentName=m.StudentName, g = n }).SelectMany(x => x.g.DefaultIfEmpty(), (x, y) => new
                {
                    StudentId = x.stu.StudentId,
                    StudentName = x.stu.StudentName,
                    StandardId = x.stu.StandardId,
                    StandardName =x.StudentName,
                    GradeName=y!=null?y.GradeName:"无年级值"
                });

            foreach (var item in list4)
            {
                StringBuilder sb = new StringBuilder();
                sb.Append("学生ID:" + item.StudentId);
                sb.Append(",姓名：" + item.StudentName);
                sb.Append(",水平ID：" + item.StandardId);
                sb.Append(",水平：" + item.StandardName);
                sb.Append(",班级：" + item.GradeName);
                Console.WriteLine(sb.ToString());
            }
            Console.ReadKey();
        }
    }
    #endregion
}

结果：

 

示例：分页查询

var page = 1;
            var pageSize = 10;
            var query = (from user in db.Set<User>()
                         join userRole in db.Set<UserRole>() on user.Id equals userRole.UserId
                         join rolePrivilege in db.Set<RolePrivilege>() on userRole.RoleId equals rolePrivilege.RoleId
                         join priviege in db.Set<Privilege>() on rolePrivilege.PrivilegeId equals priviege.Id
                         join role in db.Set<Role>() on userRole.RoleId equals role.Id
                         where user.Id == 1 && userRole.RoleId == 1
                         orderby user.Id descending
                         select new
                         {
                             user.Id,
                             userRole.RoleId,
                             user.Username,
                             PrivilegeName = priviege.Name,
                             RoleName = role.Name
                         }).Skip((page - 1) * pageSize).Take(pageSize);

（参考）Linq 多表连接查询join