Before and after triggers

just can somebody clarify
when we say
create or replace trigger tn before/after insert/update/delete for each row/statement
i just want to know what i can do with a before trigger which i cannot do with a after trigger and viceversa

<BLOCKQUOTE><font size="1" face="Verdana, Arial, Helvetica">quote:</font><HR>Originally posted by sushant prabhu:
i just want to know what i can do with a before trigger which i cannot do with a after trigger and viceversa<HR></BLOCKQUOTE>
Before row triggers can modify column values (:new.column := value) before the row is originally written to disk. I don't think after row triggers can modify :new values. Before row triggers can't see the row's rowid, at least not for inserts, but after row triggers always can.
Statement triggers can modify the triggering table, but row triggers can't. Statement triggers can't tell what rows are touched, but row triggers are called once per row touched and can see :old and :new values for that row.
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