Thank you for your reply. Then, I have two further questions on this: 1. Why this ''imports-from-a country itself'' only happens after around 2001? You may check the data, there is no such problem for early years. 2. I've noticed that your export data acturally includes ''re-exports'' as well, why there is no such problem for exports at all? ...

No, there is nothing wrong with database. It is possible that country imports from itself (re-imports). See http://comtrade.un.org/kb/article.aspx?id=10038

Re-import means that the country import the products that were manufactured/produced in the country itself. The products might be out of country for minor activities, such as packaging.

There are several possible reasons:  1. China uses Special Trade System (please refer to Concepts and Definitions of IMTS publication for further information about Trade System, see knowledge base), which means that China has free zones. These free zones are normally not part of customs territories and in special trade system trade between ...