adding rows in datagrid

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C#

WPF

DataGrid

I am developing a very basic epos type application where once the user adds a barcode and quantity into texbox and clicks a button, the datagrid shows the information added. At the moment the code works fine until I come to adding a new sale to datagrid. When I click the button, it over-rights the first recorded added to datagrid. How do I get all sales to show when I add them to datagrid?

Code so far:

private void button1_Click(object sender, RoutedEventArgs e)
        {
            MySqlConnection conn = new MySqlConnection(connectionSQL);
            
           conn.Open();

            DataSet ds = new DataSet();

            MySqlDataAdapter da = new MySqlDataAdapter("Select barcode, name, price from Product WHERE barcode='" + textBox1.Text + "'", conn);

            MySqlCommandBuilder cmd = new MySqlCommandBuilder(da);

            da.Fill(ds);
            ds.Tables[0].Columns.Add(new DataColumn("Quantity"));

            for (int i = 0; i <= ds.Tables[0].Rows.Count - 1; i++)
            {
                ds.Tables[0].Rows[i]["Quantity"] = textBox2.Text;
            }

            this.dataGrid1.ItemsSource = ds.Tables[0].DefaultView;
            conn.Close();