Bind to drop down list by passing parameter from another dropdown on selected index change

I have to drop down lists and on the first one I bind date coming from database. The second drop down list control has to be populated based on the selected index on the first one.
For example if the selection is changed on the first one the second drop down binding has to be triggered and query data from different database based on the parameter  passed from drop down list number one.
 
Can anyone help with a starter code or sample.
 
Thanks in advance. 
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