I would not create a link field.
If your datas have facts in like revenue, amount or piecec you can concatenate them.
load * from yourfile1.qvd;
concateante (Datas) load * from yourfile2.qvd;
concateante (Datas) load * from yourfile3.qvd;
and so on.
All common fields (date, order no, customer no, material no) are in the same field and can be compared with other values.
Those, who have not all fields filled (e.g. a store number which is not valid in a bill) could only be compared with those fields which are filled.
if you have common fields in diffenrent tables with same input but different sources you could not concatenate.
your invoice table have
and your customer table have also
number and name
and with material it is also
you have multiple information to one dataset.
In that case you have to decide which information (material name) is the right one. From your invoice table (the historical name, based on timestamp, the invoice was created) or from your material master (the actual name)
Also with other common fields.
A screeshot from your table view could help.