MySQL sequential processing

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I'm trying to process a series of records sequentially using a routine
something like this using Delphi 7/Zeos Components

# retrieve result set:
Query.SQL.Clear;
Query.SQL.Add(' SELECT * FROM items WHERE invoice = :invoicenumber ');
Query.Open;

# process each record of result set
WHILE (invoicenumber = item.invoice) DO
BEGIN
Query.SQL.Clear;
Query.SQL.Add(' UPDATE items SET shipped = :shipped WHERE invoice =
:invoice ');
Query.ExecSQL;
Query.Next;
END;

When reaching Query.Next, I get error that says I cannot perform on closed
dataset

Can someone tell me how to access and then update a result set sequentially?

Thanks,

Arnie