Question

I have two tables. One user table and one table in which results are placed of a certain test.

When a user takes a test his result and his user_id are placed in the results table. When a user never took a test then obviously his user_id won’t be in the results table.

I need to get all the users that are NOT in the results table. Is there a way to do this in one query? (Entity Framework)

Getting all the users is easy. But now i need a way to link it to the results table to see which of the users i want in my result set.

(from u in entity.Users
[where not in results table..??]
select u); 

Answer 1

Fully working example, using mock objects:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {

            var usersTakenTest = new List<string>() { "Bob", "Jim", "Angel" };
            var allUsers = new List<string> { "Bob", "Jim", "Angel", "Mike", "JimBobHouse" };

            var users = from user in allUsers
                        join userTakenTest in usersTakenTest on user equals userTakenTest into tempUsers
                        from newUsers in tempUsers.DefaultIfEmpty()
                        where string.IsNullOrEmpty(newUsers)
                        select user;

            foreach (var user in users)
            {
                Console.WriteLine("This user has not taken their test: " + user);
            }
            Console.ReadLine();
        }
    }
}

.DefaultIfEmpty() is what you are after – if it brings back an empty result you have an object in table A that does not show in table B.