C# 如何使用linq获取相关数据？

我有一张人员表和人员驾驶执照表。PersonnelDrivingLicense表中有多条记录与一名人员相关

我试图用linq获取人员和人员驾驶许可证数据，但我得到的是2条记录，而不是1条记录

以下是我的linq查询：

from p in Personnel
join pdl in PersonnelDrivingLicense on p.Id equals pdl.PersonnelId
select new Personnel
{
    Id = p.Id,
    PersonnelDrivingLicense = new List<PersonnelDrivingLicense>
    {
        new PersonnelDrivinLicense
        {
             Id = pdl.Id,
             DrivingLicenseClass = pdl.DrivingLicenseClass
        }
    }
}

正确的结果应如下所示：

Id: 1,
PersonnelDrivingLicense:
    Id: 1,
    DrivingLicenseClass: B

    Id: 2,
    DrivingLicenseClass: C

我怎样才能得到上面想要的结果？ 我应该如何编写正确的linq查询

---

谢谢。

您应该按Id对返回的人员对象进行分组。请尝试以下操作：

from p in Personnel
join pdl in PersonnelDrivingLicense on p.Id equals pdl.PersonnelId
group p by p.Id into g
select new Personnel
{
    Id = g.Key,
    PersonnelDrivingLicense = g.Select(x => x.PersonnelDrivingLicense).ToList()
}

---

如果您有一对多或多对多关系，并且您想要一个项目及其子项目，则应使用GroupJoin而不是Join

在您的情况下，您有

人员

和

人员驾驶执照

，可能是一对多关系：每个

人员

对象都有零个或多个

人员驾驶执照

。每个

PersonnelDrivingLicense

只属于一个

Personnel

对象，即外键

PersonnelId

所指的

Personnel

显然，您需要一系列

人员

对象，每个

人员

对象都有他/她的

人员驾驶执照

列表

// GroupJoin Personnel with PersonnelDrivingLicenses
var personnelWithTheirDrivingLicenses = myDbContext.Personnel
    .GroupJoin(myDbDcontext.PersonnelDrivingLicenses,

    // from every personnel object take the Id,
    personnel => personnel.Id,
    // from every driving license take the PersonnelId
    personnelDrivingLicence => personnelDrivingLicence.PersonnelId,

    // Take the Personnel, with all his matching driving licenses to make a new:
    (personnel, drivingLicenses) => new
    {
        // for optimal efficiency, Select only the properties you plan to use:
        Id = personnel.Id,
        Name = personnel.Name,
        ...

        DrivingLicenses = drivingLicenses
            .Where(drivingLicense => ...) // only if you don't want all his DrivingLicenses
            .Select(drivingLicense => new
            {
                // again, select only the properties you plan to use:
                Id = drivingLicense.Id,
                Type = drivingLicense.Type,
                ...

                // not needed, you already know the value:
                // PersonnelId = drivingLicense.PersonnelId,
            })
            .ToList(),
    });

---

如果您在人员和驾驶执照表（驾驶执照收集的导航属性）之间存在关系：

context.personal.Include（p=>p.DrivingLicenses）。ToList（）

（内部）连接生成一个叉积结果，基本上每个连接键匹配一行。这就是为什么你会看到“重复”的结果。通过对行进行分组，可以解决此问题。此外，如果您使用的是实体框架，那么根据您的模型关系，有更简单的方法来实现这一点。您可以将实体模型添加到您的问题中吗？

// GroupJoin Personnel with PersonnelDrivingLicenses
var personnelWithTheirDrivingLicenses = myDbContext.Personnel
    .GroupJoin(myDbDcontext.PersonnelDrivingLicenses,

    // from every personnel object take the Id,
    personnel => personnel.Id,
    // from every driving license take the PersonnelId
    personnelDrivingLicence => personnelDrivingLicence.PersonnelId,

    // Take the Personnel, with all his matching driving licenses to make a new:
    (personnel, drivingLicenses) => new
    {
        // for optimal efficiency, Select only the properties you plan to use:
        Id = personnel.Id,
        Name = personnel.Name,
        ...

        DrivingLicenses = drivingLicenses
            .Where(drivingLicense => ...) // only if you don't want all his DrivingLicenses
            .Select(drivingLicense => new
            {
                // again, select only the properties you plan to use:
                Id = drivingLicense.Id,
                Type = drivingLicense.Type,
                ...

                // not needed, you already know the value:
                // PersonnelId = drivingLicense.PersonnelId,
            })
            .ToList(),
    });