How to populate a combobox on the selection of another combo

Dear Sir i need this type of article

Yes. Riddhi is correct. But to ad that you can fetch all the data I mean for both the drop down at the time of page load and store it. And as she said on selection change of dropdown1 you can databound dropdown2 putting  condition. This will save your time and you don't need to fetch the data again from DB for dropdown2.