public static int AddReader(string login, string password, string name, string surname, string city, string birthDate, string phoneNumber)
        {
            int readerID;
            string query = $"INSERT INTO Reader OUTPUT INSERTED.ID VALUES('{login}', '{password}', '{name}', '{surname}', @city, @birthDate, @phoneNumber)";

            using (SqlConnection connection = new SqlConnection(Program.ConnectionString))
            using (SqlCommand command = new SqlCommand(query, connection))
            {
                connection.Open();

                if (city != null)
                    command.Parameters.AddWithValue("@city", city);
                else
                    command.Parameters.AddWithValue("@city", DBNull.Value);

                if (birthDate != null)
                    command.Parameters.AddWithValue("@birthDate", birthDate);
                else
                    command.Parameters.AddWithValue("@birthDate", DBNull.Value);

                if (phoneNumber != null)
                    command.Parameters.AddWithValue("@phoneNumber", phoneNumber);
                else
                    command.Parameters.AddWithValue("@phoneNumber", DBNull.Value);

                readerID = (int)command.ExecuteScalar();
            }

            return readerID;
        }