Invoice rownumbers (tdataset), where am I

it depends on where you want this ... and where you want it stored
and what type of database you are using

typically it's done at insert of the record
> using a trigger with a sequence number or autonumber field

for detail records, it's not really required
it can be generated with a certain sort order

sample select with generated row numbers per invoice

select 
  row_number() over (partition by invoice_id order by article, quantity) as detail_row, 
  d.*
from invoice_details d 

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