Maybe one exists already but I haven't seen one....but it would be good to have a tutorial for more advanced aggregate queries. For example see the information in the following discussions, all which contain different information not included in the main web app tutorial at least:
https://www.outsystems.com/forums/discussion/36045/filter-by-count-group-by/
https://www.outsystems.com/forums/discussion/35598/group-by/
https://www.outsystems.com/forums/discussion/38986/count-a-groupby/