Marking Footers in a tablr

[Michael Smith]

I have a table of invoices that are identified uniquely by their Invnbr,

I would like to mark the end of each group with EXTRA1 = "X".

I have tried to use code like:

UPDATE Invoices01 GROUP by Invnbr set EXTRA1 = "X" ;

This gives me an error message.

How would I do this?

Mike