How to check if a value already exists in my database or checking user name availability

(Note: I have table column name matno with some values like SB-1003,SB-1004.....etc )

See demo video:

Screen Shot 1:

  

Screen Shot 2:

Screen Shot 3:

Aspx Code :

(for Text box and Label and Required Field Validator):-

 </td><td>

                    &nbsp;</td><td class="style3">Material Reference No</td><td class="style5">

        <asp:TextBox ID="matnotxt" runat="server"  CssClass="textbox" Height="25px" 

            Width="200px" ontextchanged="txtmatno_TextChanged"  AutoPostBack="True" ></asp:TextBox>

    </td><td>

        <asp:RequiredFieldValidator ID="RequiredFieldValidator15" runat="server" 

            ControlToValidate="matnotxt" ErrorMessage="*" ForeColor="#CC0000"></asp:RequiredFieldValidator>

    </td><td>&nbsp;</td></tr>

<asp:Label ID="lbler" runat="server" ForeColor="#CC0000" Text="Label"></asp:Label>

C# Code :

(For text box changed event):-

    protected void txtmatno_TextChanged(object sender, EventArgs e)

    {

        if (Page.IsPostBack == true)

        {

        string connString = ConfigurationManager.ConnectionStrings["constr1"].ConnectionString;

        MySqlConnection myConnection = new MySqlConnection(connString);

        using (var cmd = new MySqlCommand("select matno from sbm where matno=@matno", myConnection))

        {

            myConnection.Open();

            cmd.Parameters.AddWithValue("@matno", matnotxt.Text);

            using (var dr = cmd.ExecuteReader())

            {

                if (dr.HasRows)

                {

                    lbler.Visible = true;

                    lbler.Text = "already exists";

                }

                else

                {

                    lbler.Visible = true;

                    lbler.Text = "doesn't exists";

                }

            }

        }

        myConnection.Close();

    }

}