Filling Registration Form Fields dynamicalle

I would like to see how i can fill a field automatically by querying the database based on a selection made by the user on a previous field.
For example I have two fields entitled category1 and category2 , which are both query drop down fields. The first has the same set of data whereas the second shows different options depending on the choice made in category1.

Any idea how to implement that using CB plugins. What are the plugins to use and how to pass the parameter indicating the first choice to the second field ?

We've no plugin available to do that. CB Query Field has no ajax features except for validation its Query Validation, which doesn't do what you're wanting. You'd need to modify the plugin and implement additional ajax usage.

Thank you for your reply. Would you recommend any tutorials or documentation that would help with respect to plugin development.
Furthermore, which plugin do I need to modify, the CB Query ?

Thanks.

Would you recommend any tutorials or documentation that would help with respect to plugin development.

No, your usage is too specific to document except for the Ajax endpoint, which CB Query Field already has for its validation.

Furthermore, which plugin do I need to modify, the CB Query ?

Yes, you'd need to implement for it to refresh it self when a specific field changes or maybe when any value in the form changes. You'd need to write custom jQuery for that then add a new function handling IF to the Ajax endpoint (function fieldClass) for the field.