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populate dropdown list

I need to populate a dropdown list on the basis of other dropdown list. Both dropdwon list are datasourced with SQL datasource. I tried the SQL datasource where query . but it will need a post back. I dont want my form to be postback. I guess this is what AJAX is for but i really dont know how to get this with AJAX in a simple and quick way..... or it is not simple and quick ????