How to deduct quantity from stock C# sql
<p>Once the product is bought it should reduce from stock quantity</p>

<pre><code>double qun;

qun = Convert.ToDouble(dataGridView1.Rows[0].Cells[3].Value) - Convert.ToDouble(textBox2.Text);

sqlconnection = new SqlCeConnection(ConnectionString);
sqlcommand = new SqlCeCommand();
sqlconnection.Open();
sqlcommand.Connection = sqlconnection;
sqlcommand.CommandText = (@"UPDATE ItemStock_Info SET Quantity =@qun WHERE [Item_Number]='"+ textBox1.Text +"'");
sqlcommand.Parameters.Add("@qun", qun);

sqlcommand.ExecuteNonQuery();
sqlconnection.Close();
</code></pre>