Who can help 

Hi again kevin.

Alright. The thing here is that you cannot connect different tables when their structure isn't the same.

For instance - your tables for villavolta and coolboy93 look like this:

Datum | Bier | Wijn | Wodka | Whiskey | Water Fles | Boekje | Hoeveel Geld

While the table you had in mind of making a cash register looks like this:

Producht | Prijs | Ingekocht | Verkocht | Vooraad

So, when you insert a number in column "Bier", it can be assigned to another column in the other table, but you're asking for conditional logic where the plugin would add it to another column + a specified row. It cannot do that.

If you know your way around SQL, you may be able to do that using PIVOT function, but please note that this doesn't fall under included support of the plugin.

Best regards.