How to create a customized Report?

Hello Team,

I would like to create some specific reports 1st for administration purpose and in a 2cd step, for the user.

For instance :
- list of Events not linked to a Group
- list of Events with the list off all Attendees
- list of Users with their Events they attend
- etc.

Can you provide me a sample of code in order to display a web page + a very basic SQL request.
It will be a good starting point for me and I can change the SQL request to fit my needs.

Thanks in advance
Henri