private void AddClientButton_Click(object sender, RoutedEventArgs e)
        {
            try
            {
                int    id         = Convert.ToInt32(ID_textbox.Text);
                string surname    = Surname_textbox.Text;
                string name       = Name_textbox.Text;
                string patronymic = Patronymic_textbox.Text;
                string phone      = Phone_textbox.Text;
                string data       = DataBirth_textbox.Text;
                string adres      = Address_textbox.Text;
                string login      = Login_textbox.Text;
                string password   = Password_textbox.Text;

                string        connectionString = @"Data Source =.\SQLEXPRESS; Initial Catalog = FitnessCenter; Integrated Security = True";
                SqlConnection connection       = new SqlConnection(connectionString);
                connection.Open();
                string     cmdTxt  = $"INSERT INTO Clients (ID_client, Surname, Name, Patronymic, PhoneNumber, DateOfBirth, Address, Login, Password) VALUES ('{id}', '{surname}', '{name}', '{patronymic}', '{phone}', '{data}', '{adres}', '{login}', '{password}')";
                SqlCommand command = new SqlCommand(cmdTxt, connection);
                int        number  = command.ExecuteNonQuery();
                MessageBox.Show("Изменения сохранены успешно!\nВставлено объектов: " + number, "Статус действия", MessageBoxButton.OK, MessageBoxImage.Information);

                ID_textbox.Clear(); Surname_textbox.Clear(); Name_textbox.Clear(); Patronymic_textbox.Clear();
                Phone_textbox.Clear(); DataBirth_textbox.Clear(); Address_textbox.Clear(); Login_textbox.Clear();
                Password_textbox.Clear();
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message, "ERROR!", MessageBoxButton.OK, MessageBoxImage.Error);
            }
        }
        private void ChangeClientButton_Click(object sender, RoutedEventArgs e)
        {
            try
            {
                int    old_id         = Convert.ToInt32(ID_textbox.Text);
                string old_surname    = Surname_textbox.Text;
                string old_name       = Name_textbox.Text;
                string old_patronymic = Patronymic_textbox.Text;
                string old_phone      = Phone_textbox.Text;
                string old_data       = DataBirth_textbox.Text;
                string old_adres      = Address_textbox.Text;
                string old_login      = Login_textbox.Text;
                string old_password   = Password_textbox.Text;

                int    id         = Convert.ToInt32(NewID_textbox.Text);
                string surname    = NewSurname_textbox.Text;
                string name       = NewName_textbox.Text;
                string patronymic = NewPatronymic_textbox.Text;
                string phone      = Phone_textbox.Text;
                string data       = NewDateBirth_textbox.Text;
                string adres      = NewAddress_textbox.Text;


                string        connectionString = @"Data Source =.\SQLEXPRESS; Initial Catalog = FitnessCenter; Integrated Security = True";
                SqlConnection connection       = new SqlConnection(connectionString);
                connection.Open();

                string     cmdTxt1  = $"UPDATE Clients SET Surname = '{surname}'  WHERE ID_client = '{old_id}'";
                SqlCommand command1 = new SqlCommand(cmdTxt1, connection);
                string     cmdTxt2  = $"UPDATE Clients SET Name = '{name}'  WHERE ID_client = '{old_id}'";
                SqlCommand command2 = new SqlCommand(cmdTxt2, connection);
                string     cmdTxt3  = $"UPDATE Clients SET Patronymic = '{patronymic}'  WHERE ID_client = '{old_id}'";
                SqlCommand command3 = new SqlCommand(cmdTxt3, connection);
                string     cmdTxt4  = $"UPDATE Clients SET PhoneNumber = '{phone}'  WHERE ID_client = '{old_id}'";
                SqlCommand command4 = new SqlCommand(cmdTxt4, connection);
                string     cmdTxt5  = $"UPDATE Clients SET DateOfBirth = '{data}'  WHERE ID_client = '{old_id}'";
                SqlCommand command5 = new SqlCommand(cmdTxt5, connection);
                string     cmdTxt6  = $"UPDATE Clients SET Address = '{adres}'  WHERE ID_client = '{old_id}'";
                SqlCommand command6 = new SqlCommand(cmdTxt6, connection);

                command1.ExecuteNonQuery(); command2.ExecuteNonQuery(); command3.ExecuteNonQuery(); command4.ExecuteNonQuery(); command5.ExecuteNonQuery(); command6.ExecuteNonQuery();
                MessageBox.Show("Изменения сохранены успешно!", "Статус действия", MessageBoxButton.OK, MessageBoxImage.Information);


                ID_textbox.Clear(); Surname_textbox.Clear(); Name_textbox.Clear(); Patronymic_textbox.Clear();
                Phone_textbox.Clear(); DataBirth_textbox.Clear(); Address_textbox.Clear(); Login_textbox.Clear();
                Password_textbox.Clear();
                NewID_textbox.Clear(); NewSurname_textbox.Clear(); NewName_textbox.Clear(); NewPatronymic_textbox.Clear();
                NewPhone_textbox.Clear(); NewDateBirth_textbox.Clear(); NewAddress_textbox.Clear();
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message, "ERROR!", MessageBoxButton.OK, MessageBoxImage.Error);
            }
        }
        private void DeleteClientButton_Click(object sender, RoutedEventArgs e)
        {
            try
            {
                int id = Convert.ToInt32(ID_textbox.Text);

                string        connectionString = @"Data Source =.\SQLEXPRESS; Initial Catalog = FitnessCenter; Integrated Security = True";
                SqlConnection connection       = new SqlConnection(connectionString);
                connection.Open();
                string     cmdTxt  = $"DELETE FROM Clients WHERE ID_client = '{id}'";
                SqlCommand command = new SqlCommand(cmdTxt, connection);
                int        number  = command.ExecuteNonQuery();
                MessageBox.Show("Изменения сохранены успешно!\nУдалено объектов: " + number, "Статус действия", MessageBoxButton.OK, MessageBoxImage.Information);

                ID_textbox.Clear(); Surname_textbox.Clear(); Name_textbox.Clear(); Patronymic_textbox.Clear();
                Phone_textbox.Clear(); DataBirth_textbox.Clear(); Address_textbox.Clear(); Login_textbox.Clear();
                Password_textbox.Clear();
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message, "ERROR!", MessageBoxButton.OK, MessageBoxImage.Error);
            }
        }