import export in oracle

http://www-it.desy.de/systems/services/databases/oracle/impexp/impexp.html.en

http://www.orafaq.com/wiki/Import_Export_FAQ

What are the common Import/ Export problems?
ORA-00001: Unique constraint (...) violated
You are importing duplicate rows. Use IGNORE=NO to skip tables that already exist (imp will give an error if the object is re-created).
ORA-01555: Snapshot too old
Ask your users to STOP working while you are exporting or use parameter CONSISTENT=YES
ORA-01562: Failed to extend rollback segment
Create bigger rollback segments or set parameter COMMIT=Y while importing
IMP-00015: Statement failed ... object already exists...
Use the IGNORE=Y import parameter to ignore these errors, but be careful as you might end up with duplicate rows.