I am working with your system since 2.99.
Currently I use Version 4.1.46.
Till now I was only selling drinks, so WPR was easy.
But now I want to cooperate with neighbor Restaurants because we don´t have a kitchen.
The idea is that our guests can order food from the Restaurants and pay in our place and we will pass the money at the end of the day to the Restaurants.
To make it easy for the guest and our waiter I have put the Food Dishes into our Menu so there will be one Bill only.
The Problem now is that the Foods are not allowed to be displayed in the WPR otherwise we have to pay taxes for the Restaurant.
I am not the person to ask on SQL yet. I have not studied it enough myself yet. I will, but right now I am focused on other areas of developing my system.
@emre or @gerlandog are good sources… there is already A LOT of information out there to get you started most of it is referenced in the article I gave you above.
SQL Is very simple language to script… it doesnt change much its pretty straight forward. Once you learn the basics like Joins and formatting you will be able to do most of what your looking for. You can even download SQL Visual Editors that can design scripts for you.