allow to reset database

Database.cs

@@ -17,7 +17,7 @@ namespace it_projekt
             {
                 connection.Open();
 
-                string sql = "SELECT stammdaten.id, stammdaten.firstname, stammdaten.lastname, stammdaten.creationdate, abteilung.kuerzel, abteilung.name FROM `stammdaten` JOIN abteilung GROUP BY stammdaten.id; ";
+                string sql = "SELECT stammdaten.id,stammdaten.firstname, stammdaten.lastname, stammdaten.creationdate, abteilung.kuerzel, abteilung.name FROM `stammdaten` JOIN abteilung WHERE abteilungs_id = abteilung.id;";
                 MySqlCommand cmd = new MySqlCommand(sql, connection);
                 MySqlDataReader rdr = cmd.ExecuteReader();
                 Persons = new List<Person>();
@@ -33,9 +33,7 @@ namespace it_projekt
             {
                 Console.WriteLine(ex.ToString());
             }
-
             connection.Close();
-            Console.WriteLine("Done.");
         }
 
         public static Database getDatabase(string username, string password)
@@ -46,5 +44,10 @@ namespace it_projekt
             }
             return _database;
         }
+
+        public static void resetDatabase()
+        {
+            _database = null;
+        }
     }
 }

Form1.cs

@@ -39,6 +39,7 @@ namespace it_projekt
             db = Database.getDatabase(user, password);
             if (db.Persons == null)
             {
+                Database.resetDatabase();
                 MessageBox.Show("Connection to Database failed! \nPlease Try again.", "Database Error!");
             }
             else {