如何使用计数在四个表之间获取 Linq 中的数据到 EF 连接？

Question

我正在尝试使用 Linq to Ef 获取数据我是 Linq 的新手，

我有桌子，

*region having region_id , region_name
*governorate having governorate_id,governorate_name, region_id
*Student having id,governorate_id
*Teachers having id,governorate_id

我想通过使用一个 linq to EF 查询来查找每个区域下的学生和教师的数量

我尝试在 sql 查询中这样做，就像只针对学生，

select R.region_id,R.region_name , count(IR.Id) as Individual from region R 
inner join governorate G
           on r.region_id=g.region_id
inner join Student IR
            on g.governorate_id=IR.governorate_id
group by R.region_id,R.region_name

希望您的建议

Answer 1

由于您也使用 Ef，我建议您正确设置实体之间的关系，这样您就不需要自己构建连接查询（并使用 Include 子句）。

例如：

public class Region
{
    public int Id { get; set; }
    public string Name { get; set; }

    public ICollection<Governorate> Governorates { get; set; }
}

public class Governorate
{
    public int Id { get; set; }
    public string Name { get; set; }

    public int RegionId { get; set; }
    public Region Region { get; set; }

    public ICollection<Student> Students { get; set; }
}

public class Student
{
    public int Id { get; set; }

    public int GovernorateId { get; set; }
    public Governorate Governorate { get; set; }
}

希望对你有帮助

更新一个可能对您有帮助的查询：

var result = whatever.Regions
            .Include(r => r.Governorates)
            .ThenInclude(g => g.Students)
            .GroupBy(r => new {r.Id, r.Name})
            .Select(g => new
            {
                g.Key.Id,
                g.Key.Name,
                Individual = g.Count(r => r.Id)
            }).ToList();

问候

Answer 2

在一个查询中计算学生和教师：

var teachersStudents =
    (
        from r in db.Region
        join g in db.Governorate on r.region_id equals g.region_id
        join s in db.Student on g.governorate_id equals s.governorate_id
        join t in db.Teacher on g.governorate_id equals t.governorate_id
        group new { r, s, t } by new { r.region_id, r.region_name }
        into grp
        select new
        {
            grp.Key.region_id,
            grp.Key.region_name,
            CountStudents = grp.Select(s => s.s.id).Distinct().Count(),
            CountTeachers = grp.Select(t => t.t.id).Distinct().Count()
        }
    ).ToList();