problema creating a trigger in oracle to update other table - oracle11g

Question

i'm creating a trigger to update other table to audit.
My main table that is being audited is "employed" and the audit table is "salary_history". Every a update is made in emplyed table the salary_history must be updated.
I'm working with this trigger in oracle 11g:
CREATE OR REPLACE TRIGGER salary_auditing
AFTER UPDATE ON employed
REFERENCING NEW AS NEW OLD AS OLD
BEGIN
INSERT INTO SALARY_HISTORY(ID, EMPLOYED_ID, OLD_SALARY, NEW_SALARY, DATE_MODIFIED) VALUES (SELECT MAX(ID)+1 FROM SALARY_HISTORY, ID, :OLD.salary, :NEW.salary, SYSDATE)
END salary_auditing
The problema is that when i run this trigger i receive this error:
Relatório de erros -
ORA-04082: referências NEW ou OLD não permitidas nos gatilhos de nível de tabela
04082. 00000 - "NEW or OLD references not allowed in table level triggers"
*Cause: The trigger is accessing "new" or "old" values in a table trigger.
*Action: Remove any new or old references.
In english it means that the NEW or OLD values is not allowed at table nivel.
what modifications i should do?

Answer 1

Oracle has row-level and statement-level triggers. Only row-level triggers have access to the :new and :old pseudorecords. Your trigger is currently defined as a statement-level trigger. You need to add FOR EACH ROW to make it a row-level trigger.
Getting the max(id)+1 in an insert statement is a really really bad idea. Unless you have a single user application, that will cause multiple sessions to get the same value and lead to duplicate key errors. Create a sequence and use that sequence.
Saying REFERENCING NEW AS NEW OLD AS OLD is pointless. Unless you are intending to provide different aliases for the :new and :old pseudo-records, omit that. And you should almost never use different aliases.
I would expect that you want
create sequence seq_salary_history_id
start with <<value larger than what is currently in the table>>;
CREATE OR REPLACE TRIGGER salary_auditing
AFTER UPDATE ON employed
FOR EACH ROW
BEGIN
INSERT INTO SALARY_HISTORY(ID,
EMPLOYED_ID,
OLD_SALARY,
NEW_SALARY,
DATE_MODIFIED)
VALUES (seq_salary_history_id.nextval,
:new.ID,
:OLD.salary,
:NEW.salary,
SYSDATE);
END salary_auditing;
If you only want the trigger to fire when you are updating the salary column (since that's the only thing you're tracking history on), add an OF salary to your trigger declaration
CREATE OR REPLACE TRIGGER salary_auditing
AFTER UPDATE OF salary ON employed
FOR EACH ROW