Master/Detail Filtering With a DropDownList

I was looking  tutorial for Master/Detail and I have one question. If I populate DropDowList with some data from database and run application, DropDownList show the first record from database. And if I choose some Item from DropDownList DataList show result of filtering. How when I run application to populate DataList as Detail using parametar of first Item in DropDownList populated from database?
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