EntityFramework (.NET Core) 通过多个条件进行左外连接

使用EntityFramework (.NET Core)进行多条件的左外连接

// 定义实体类
public class Student
{
    public int Id { get; set; }
    public string Name { get; set; }
}
public class Course
{
    public int Id { get; set; }
    public string Name { get; set; }
    public int StudentId { get; set; }
}
// 定义数据库上下文类
public class SchoolContext : DbContext
{
    public DbSet Students { get; set; }
    public DbSet Courses { get; set; }
    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        optionsBuilder.UseSqlServer("连接字符串");
    }
}
// 查询左外连接
using (var context = new SchoolContext())
{
    var query = from student in context.Students
                join course in context.Courses on student.Id equals course.StudentId into gj
                from subcourse in gj.DefaultIfEmpty()
                select new { student.Name, CourseName = (subcourse == null ? "未选课" : subcourse.Name) };
    foreach (var item in query)
    {
        Console.WriteLine($"学生姓名：{item.Name}，课程名称：{item.CourseName}");
    }
}