limiting the selection of a record to update in SQL

Hello all,

We are trying to update a file where we have records where the only thing that 
MAY be different are the values(in this case number of items picked) and dates, 
so what we are trying to do is get SQL to read only one record at a time and 
then update a flag so it is not read again, then do some other processing 
before the next record is read. We cannot find out how to limit SQL to only 
update one record at a time. Can anyone help?

TIA

Steve Raby