Turn on DDL auditing

How to audit DDL statements in oracle 10g and clear audited data before 30days... Thank you.

IN EDITION:

How can I turn on auditing on "ALTER TABLE" statement? when I run AUDIT TABLE; or AUDIT ALTER TABLE; or AUDIT ALTER ANY TABLE; Audited information can'开发者_如何学JAVAt be seen in AUD$ or DBA_AUDIT_TRAIL views.(Just event when system user alters table is written,but there is no record for HR user) why????

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http://www.oracle-base.com/articles/10g/Auditing_10gR2.php

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Check if the audit option is enabled:

 select value
 from v$parameter
 where name = 'audit_trail';

if the result is 'NONE' (which is default) then the audit is not turned on.

To enable audit you must set the AUDIT_TRAIL initialization parameter in the database initialization parameter file. The parameter must be set to one of the following values: 'DB', 'XML', 'DB,EXTENDED', 'XML,EXTENDED', 'OS'.

For managing audit trail records you can use the DBMS_AUDIT_MGMT package.