For training you should really use SambaPOS training mode (search it).
Using SQL to remove selected data is a dangerous move. Not so much in V2, but V3 has interlocked tables. And inventory is worse. When I wrote the bulk import script it took days to get it right, and I also looked at a "reset" script to reset inventory back to imported levels, and this was not possible at that time. Not all processing of inventory is done at End of Work Period.
Please be very careful as you could destroy data and not realise.