working with dropdowns and datasources using sharepoint designer

Question

Dear All,
I am very new to SPdesigner and stuck with some sort of task. I have database with two tables (Empdtls(empsno,empname,depsno) and departments(depsno,depname))
I want to have two dropdowns in a page where first dropdown get all department names from departmenttable using datasource and second dropdown should display employee names from table empdtls. I am successfull in displaying the values in both of these dropdowns but the problem where i am facing is the FILTER OPTION FORM DROPDOWN1 TO DROPDOWN2 USING AUTOPOSTBACK.
eg. when first dropdown (department) is selected, the second dropdown should be populated with the respective empnames.
Please help me out in this