Two droplists with one Datagrid

Hi I am building a website that has a datagrid and two combo boxes.
The information for the datagrid is reterieved from a database.
And the combo box is configured to two different fields in the database. (for example county and country)
The first dropdownlist works fin only brings back the countries that i pick for example england then all data is england.
But what I want is that when i click the second box and pick a county with the country I want the datagrid to list England and hertfordshire things only not the rest of the counties.

How do i go about do this or any there any tutorials i can follow.

Any help is much appreciated.

Thanks,

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