Hello Michel,
You can usually figure it out by matching the Foreign Key with the Primary Key of the referenced table; by looking at the data.
Sometimes the Keys are named differently from one table to another that relate. For example the 'Document Number' fields in both of the following tables:
Sales Invoice Header = [No_]
Sales Invoice Line = [Document No_]
Sometimes the Keys are named the same, for example the 'Item Number' fields in both of the following tables:
Sales Invoice Line = [No_]
Item = [No_]
However, NAV/Business Central is fairly consistent. Once you see how several of the tables relate then you'll start to notice the convention that they used, which will help you figure out more table relations.
Best Regards,
Alex A
