$criteria = new Criteria(); $criteria->addGroupByColumn('category_id');In this example, the query results will be grouped by the `category_id` column. This means that all rows with the same `category_id` value will be grouped together. `addGroupByColumn` is part of a package called `Propel`, which is an open-source Object-Relational Mapping (ORM) library for PHP. Propel is designed to simplify database interactions in PHP projects by allowing developers to work with databases using object-oriented programming principles.