You can try this..
string query = "select * from emp where salary < 1000";
cmd = new SqlCommand(query,con);
da = new SqlDataAdapter(cmd);
dt = new DataTable();
da.Fill(dt);
dropdownList1.DataSource = dt;
dropdownList1.DataValueField = "empName";
dropdownList1.DataTextField= "empName";
dropdownList1.DataBind();