try this:-
Use SessionParameter with SelectParameters to pass Session values
<asp:SqlDataSource ID="SqlDataSource2" runat="server"
ConnectionString="<%$ ConnectionStrings:yourConnection %>"
SelectCommand="SELECT * FROM [examConfig] WHERE [tchId]= @getTchId">
<SelectParameters>
<asp:SessionParameter Name="Name" SessionField="getTchId" Type="String" />
</SelectParameters>
</asp:SqlDataSource>