如何使用带有 EF 6 的 Linq 扩展方法编写子查询答案

【问题标题】：How to write sub queries using Linq extension methods with EF 6如何使用带有 EF 6 的 Linq 扩展方法编写子查询
【发布时间】：2015-02-25 11:20:48
【问题描述】：

我是 linq 新手，我有 3 个包含这些列的表。

Trainee (ID, TraineeName)
Course (ID, CourseName)
TraineeCourseEnrollment (TraineeID, CourseID, EnrolledDate)

我创建了一个存储过程来使用此查询获取未注册的课程。

select * 
from Course 
where ID not in (select CourseID 
                 from TraineeCourseEnrollment 
                 where TraineeID = @traineeid);

如何使用扩展方法将对应的linq查询写入这个SQL查询？

【问题讨论】：

linq equivalent sql query "not in (select query)"的可能重复

标签： c# linq entity-framework subquery extension-methods

【解决方案1】：

var prospectus = new [] 
{ 
    new { CourseId = "C1", CourseName = "Database" },
    new { CourseId = "C2", CourseName = "HCI" },
    new { CourseId = "C3", CourseName = "Op Systems" },
    new { CourseId = "C4", CourseName = "Programming" }
};

var enrollment = new []
{
    new { TraineeID = "T1", CourseId = "C1", Date = new DateTime(2014, 12, 01) },
    new { TraineeID = "T2", CourseId = "C1", Date = new DateTime(2014, 12, 05) },
    new { TraineeID = "T1", CourseId = "C3", Date = new DateTime(2014, 12, 01) }
};

var notMatchingQueryStyle = from c in prospectus
                            where !enrollment.Any(r => c.CourseId == r.CourseId)
                            select c;

Resharper 唠叨我用All 而不是Any 来“简化”这个，去看看：

var notMatchingQueryStyle = from c in prospectus
                            where enrollment.All(r => c.CourseId != r.CourseId)
                            select c;

【讨论】：

【解决方案2】：

类似这样的：

扩展方法：

int traineeid = ...;

var query = dc.Courses
              .Where(c => ! dc.TraineeCourseEnrollments
                              .Where(o => o.TrainessID == traineeid)
                              .Select(o => o.CourseID)
                              .Contains(c.ID));

LINQ 查询：

int traineeid = ...;

var query =    
    from c in dc.Courses
    where !(from o in dc.TraineeCourseEnrollments
            where o.TraineeID == traineeid
            select o.CourseID)    
           .Contains(c.ID)    
    select c;

【讨论】：

【解决方案3】：

您必须执行两个查询，第一个是检索您要排除的 ID，第二个是获取实际课程：

var excludeIDs = db.TraineeCourseEnrollments.Where(w => w.TraineeID == traineeid).Select(s => s.CourseID);
var courses = db.Courses.Where(w =>!excludeIDs.Contains(w.ID)).ToList();

【讨论】：