SQL "not in" syntax and nested SELECT in Entity Framework - sql

Question

I recently used Entity Framework. For elementary CRUD operations, I have no problems, but for more complicated queries, I do not know how to do these.
For example: how to write a nested select? How to use the NOT IN operator?
The query that I want to write is:
SELECT *
FROM course
WHERE idCourse NOT IN (SELECT idCourse
FROM reservation
WHERE idStudent = 'value');
I do not know where to start. Could you please give me some advice?

Answer 1

If I do not misunderstood your question, you want to know how to write the question's query as LINQ-to-Entity query.
Examples could be:
var courses = context.Courses
.Where(c => c.Reservations.All(r => r.idStudent != "value"))
.Select(c => c);
// with eager loading
var courses = (from c in context.Courses.Include(c => c.Reservations)
where c.Reservations.All(r => r.idStudent != "value")
select c).ToArray();
var courses = (from c in context.Courses
join r in context.Reservations on c.idCourse equals r.idCourse
where r => r.idStudent != "value"
select c).ToArray();
The Contains() is equivalent to EXIST IN in a query. Well, 1st and 2nd nearly the same. Only difference is the Include method to show how you could eager load data.
In the 3rd query I use the join key word to do a join operation - equivalent to an INNER JOIN. The result will contain only records where a relation between a course and a reservation exists and the searched student ID is referenced in the reservation.
If you should not use LINQ for your querys, you should take a look. It's a perfect way to decouple your data access layer from persistence layer and you could test all your queries in code too.
Here you could get a very good entry into the subject.
EDIT:
Modified example code to fit NOT IN.

Answer 2

If you are having trouble executing complex queries with LINQ, you can also execute raw SQL queries. They are easy and faster than linq queries.
Here is an example in c#
var list = dc.Database.SqlQuery<YourCourceClass>(#"SELECT *
FROM course
WHERE idCourse NOT IN(SELECT idCourse
FROM reservation
WHERE idStudent = 'value');").ToList();