How to write sub queries using Linq extension methods with EF 6

Question

I'm new to linq and I have 3 tables with these columns.

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

I created a stored procedure to get un-enrolled courses using this query.

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

How to write the corresponding linq query to this SQL query using extension methods?

Answer 1

You will have to do two queries, first to retrieve the IDs that you want to exclude and the second to get actual courses:

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

Answer 2

Something like this:

Extensions methods:

int traineeid = ...;

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

LINQ query:

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;

Answer 3

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 nags me to "simplify" this using All instead of Any , go figure:

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