If you want to develop a statisctics report, first you need to develop the SQL statement first.  If you have the SQL statement, you have the option to execute and run the SQL statement using one of the available reporting engines (jasper_report, pentaho, etc.) or you can create a view and an ORM model with _auto set to False so that it can be displayed in from the web UI.  The _auto = False will tell ORM not to create the corresponding table.  To be able to use ORM, the SQL statement need to have id column.  You can check various samples available in odoo/addons: stock/report/report_stock.py and its corresponding view stock/report/report_stock_view.xml is a pair of example.