ORA-01407: cannot update (string) to NULL

Question

I've read that one way to avoid ORA-01407 error like in this example ( [DBA-ORACLE.COM SITE|http://www.dba-oracle.com/t_ora_01407_cannot_update_string_to_null.htm] ) is to use additional EXISTS in where condition:
update ORDERS ord
  set ord.amount = ( select ord.qty * it.item_price
                     from ITEM it
                     where ord.item_id = it.item_id )
where exists ( select 1
               from ITEM it
               where ord.item_id = it.item_id );Execution plan confirms that those similar correlated subqueries are evaluated twice when I substitute 1 with select ord.qty * it.item_price to make them identical. what will happen if between those two evaluations for particular item_id if some commited update/delete happens on ITEM table causing row removal/values change for item_id? How will it influence the UPDATe? will there be any errors etc?
thank you

Answer 1

Hi,
943276 wrote:
... what will happen if between those two evaluations for particular item_id if some commited update/delete happens on ITEM table causing row removal/values change for item_id? How will it influence the UPDATe? will there be any errors etc?If you're using Oracle 4 or higher, your UPDATE statement will continue to read the table as it was the moment your statement began. If you begin the statement at 10:00, but the EXISTS sub-query is executed at 10:02, it will see data that other sessions deleted at 10:01, and it will not see data that other sessions inserted at 10:01.
Apiminov made a good point: MERGE is simpler to code and more efficient to run