var query = from student in db.Students
            join course in db.Courses on student.CourseId equals course.Id
            select new {
                student.Name,
                CourseName = course.Name
            };

var query = from student in db.Students
            join course in db.Courses on student.CourseId equals course.Id into studentCourses
            from sc in studentCourses.DefaultIfEmpty()
            select new {
                student.Name,
                CourseName = sc == null ? "No Course" : sc.Name
            };

var query = from student in db.Students
            select new {
                student.Name,
                CourseName = student.Course.Name
            };

var query = from order in db.Orders
            join customer in db.Customers on order.CustomerId equals customer.Id
            join product in db.Products on order.ProductId equals product.Id
            select new {
                OrderId = order.Id,
                CustomerName = customer.Name,
                ProductName = product.Name,
                OrderDate = order.OrderDate
            };

// 获取每个部门的员工数量及平均工资
var departmentStats = from dept in db.Departments
                      join emp in db.Employees on dept.Id equals emp.DepartmentId into deptEmps
                      select new {
                          DepartmentName = dept.Name,
                          EmployeeCount = deptEmps.Count(),
                          AvgSalary = deptEmps.Average(e => e.Salary)
                      };