Simple Example Subquery Linq

Question

T-SQL Query

Select * from dbo.User_Users
Where UserID IN (Select UserID from Course_Enrollments)

LINQ to Entities alternative of Above Quer

Answer 1

from u in User_Users
where u.Course_Enrollments.Any()
select u

if you have the foreign keys set up. If not you can do this

from u in User_Users
join cu in Course_Enrollments on u.UserId equals cu.UserId
select u

You also should wrap any of these with .Distinct() call

Answer 2

Simple answer is use the "let" keyword and generate a sub-query that supports your conditional set for the main entity.

var usersEnrolledInCourses = from u in User_Users
                                 let ces = from ce in Course_Enrollments
                                           select ce.UserID
                                 where ces.Contains(u.UserID)
                             select u;   

This will create an exists block in TSQL similar to

SELECT [Extent1].*
   FROM dbo.User_Users AS Extent1
   WHERE EXISTS (SELECT 1 AS [C1]
                     FROM dbo.Course_Enrollements AS Extent2
                     WHERE (Extent2.UserID = Extent1.UserId))

It's close to what you've asked for and will generally create the same query plan on SQL Server.

Hope this helps!