working with text boxes and drop down lists

[danholding]

hi i have a drop down list where i select an employee id, once i click the button i want it to populate the Rate text box with the rate which matches the id from the table in sql.

i have this so far but not sure if i have done it the right way and just missing code for the button or whether i am going about it the wrong way any help with this would be great!

employee ID:<asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="True" 
            DataSourceID="SqlDataSource1" DataTextField="EmployeeID" 
            DataValueField="EmployeeID">
        </asp:DropDownList>
        
        Your Pay Rate:<asp:TextBox ID="TextBox1" runat="server" DataSourceID="SqlDataSource1"></asp:TextBox>
        
        Hours Worked:<asp:DropDownList ID="DropDownList2" runat="server" 
            DataSourceID="SqlDataSource3" DataTextField="hours_worked" 
            DataValueField="hours_worked">
        </asp:DropDownList>
        
       Total Pay <asp:TextBox ID="TPay" runat="server" ReadOnly="True"></asp:TextBox><br />
        
        <asp:Button ID="SelectID" runat="server" Text="show total pay" PostBackUrl="~/Default.aspx" />


       <asp:SqlDataSource ID="SqlDataSource1" runat="server" 
            ConnectionString="<%$ ConnectionStrings:Ad_newConnectionString %>" 
            SelectCommand="SELECT EmployeeID FROM HumanResources.EmployeePayHistory">
        </asp:SqlDataSource>
        <asp:SqlDataSource ID="SqlDataSource2" runat="server" 
            ConnectionString="<%$ ConnectionStrings:Ad_newConnectionString %>" 
            SelectCommand="SELECT Rate FROM HumanResources.EmployeePayHistory WHERE EmployeeID == @DropDownList1.Text">
        </asp:SqlDataSource>
        <asp:SqlDataSource ID="SqlDataSource3" runat="server" 
            ConnectionString="<%$ ConnectionStrings:Ad_newConnectionString %>" 
            SelectCommand="SELECT [hours worked] AS hours_worked FROM [hours worked]">
        </asp:SqlDataSource>

thanks