新手求助：求对应指定多表联查SQL的DbContext实现代码

首先得确保你的实体类和DbContext配置正确，对应数据库里的三个表，示例如下：

// Students表对应的实体类
public class Student
{
    public int StudentRollno { get; set; }
    // 可添加Student表的其他字段
}

// Courses表对应的实体类
public class Course
{
    public int CourseId { get; set; }
    // 可添加Course表的其他字段
}

// AssignedCourses表对应的实体类
public class AssignedCourse
{
    public int AssignedCourseId { get; set; }
    public int StudentRollnumber { get; set; }
    // 可添加AssignedCourses表的其他字段
}

// 自定义DbContext类
public class YourDbContext : DbContext
{
    public DbSet<Student> Students { get; set; }
    public DbSet<Course> Courses { get; set; }
    public DbSet<AssignedCourse> AssignedCourses { get; set; }

    // 配置数据库连接字符串
    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        optionsBuilder.UseSqlServer("你的数据库连接字符串");
    }
}

下面是两种和你给出的SQL等效的实现方式：

方式1：LINQ查询语法（和SQL结构接近，新手易读） #

using (var dbContext = new YourDbContext())
{
    var query = from ac in dbContext.AssignedCourses
                join student in dbContext.Students on ac.StudentRollnumber equals student.StudentRollno
                join course in dbContext.Courses on ac.AssignedCourseId equals course.CourseId
                // 可按需选择字段，或直接返回包含所有表数据的匿名对象
                select new 
                {
                    AssignedCourse = ac,
                    Student = student,
                    Course = course
                };

    // 执行查询并获取结果列表
    var result = query.ToList();
}

方式2：LINQ方法语法（链式调用风格） #

using (var dbContext = new YourDbContext())
{
    var query = dbContext.AssignedCourses
        .Join(dbContext.Students,
              assignedCourse => assignedCourse.StudentRollnumber,
              student => student.StudentRollno,
              (assignedCourse, student) => new { assignedCourse, student })
        .Join(dbContext.Courses,
              combined => combined.assignedCourse.AssignedCourseId,
              course => course.CourseId,
              (combined, course) => new { combined.assignedCourse, combined.student, course });

    var result = query.ToList();
}

额外优化：用导航属性简化查询 #

如果你的实体类之间配置了导航属性（比如在AssignedCourse里添加关联的Student和Course对象），可以用Include方法直接加载关联数据，EF Core会自动生成对应的Join语句：

// 修改AssignedCourse实体类，添加导航属性
public class AssignedCourse
{
    public int AssignedCourseId { get; set; }
    public int StudentRollnumber { get; set; }
    public int AssignedCourseId { get; set; } // 对应Course的CourseId

    // 导航属性
    public Student Student { get; set; }
    public Course Course { get; set; }
}

// 查询代码
using (var dbContext = new YourDbContext())
{
    var result = dbContext.AssignedCourses
        .Include(ac => ac.Student)
        .Include(ac => ac.Course)
        .ToList();
}

内容的提问来源于stack exchange，提问作者Usman Arshad