using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var solvedExercises = new List<DatabaseEntry>

        {

            new DatabaseEntry { StudentId = 1, ExerciseId = 1 },

            new DatabaseEntry { StudentId = 1, ExerciseId = 2 },    

            new DatabaseEntry { StudentId = 2, ExerciseId = 2 },    

            new DatabaseEntry { StudentId = 3, ExerciseId = 1 },    

            new DatabaseEntry { StudentId = 3, ExerciseId = 2 },    

            new DatabaseEntry { StudentId = 3, ExerciseId = 3 },        

        };

        var result1 = solvedExercises

            .GroupBy(e => e.StudentId)

            .ToDictionary(e => e.Key, e => e.Select(e2 => e2.ExerciseId).ToList());

        foreach(var student in result1)

        {

            Console.Write($"Student #{ student.Key }: ");

            foreach(var exercise in student.Value)

                Console.Write($"{ exercise } ");

            Console.WriteLine();

        }

        Console.WriteLine();

        var result2 = solvedExercises

            .GroupBy(e => e.StudentId)

            .Select(e => new { e.Key, Count = e.Count() })

            .ToDictionary(e => e.Key, e => e.Count);

        foreach(var student in result2)

        {

            Console.WriteLine($"Student #{ student.Key }: { student.Value }");

        }

    }

}

public class DatabaseEntry

{

    public int StudentId { get; set; }

    public int ExerciseId { get; set; }

}