Explain SELECT ... FOR UPDATE and WHERE CURRENT OF

Declaring a cursor with SELECT ... FOR UPDATE [OF col] locks every selected row when the cursor opens, holding the locks until COMMIT or ROLLBACK — so keep the transaction short. Inside the loop, UPDATE/DELETE ... WHERE CURRENT OF cursor_name targets exactly the current row without repeating the key predicate. Add FOR UPDATE SKIP LOCKED to step over rows another session already locked (a simple queue pattern), or NOWAIT to fail fast instead of blocking.

DECLARE
  CURSOR c_ap IS
    SELECT invoice_id, amount
      FROM ap_invoices_all
     WHERE process_flag = 'N'
       FOR UPDATE SKIP LOCKED;
BEGIN
  FOR r IN c_ap LOOP
    UPDATE ap_invoices_all
       SET process_flag = 'Y', processed_on = SYSDATE
     WHERE CURRENT OF c_ap;
  END LOOP;
  COMMIT;
END;
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