Dear Friends,
I need some help to check avilidity of the username on Leave event of a textbox in Database SQL Server, but i cann't get.
it code is bellow

private void textBox6_Leave(object sender, EventArgs e)
            cmd = new SqlCommand("SELECT * FROM usertab WHERE username='"+textBox6.text+"'",con);
            SqlDataAdapter da = new SqlDataAdapter(cmd);

            int i = ds.Tables[0].Rows.Count;
            if (i > 0)
                label21.Text = "username is already exists";

                label21.Text = "username is available";


con Connetion and DataSet=ds are defined in public section.

SqlConnection con = new SqlConnection("data source=MUHAMMAD\\SQLEXPRESS; database = MMS; integrated security = SSPI");
DataSet ds = new DataSet();