This article is useful for people who new with administrating Oracle database. When you've done something wrong with schemas or tables and you want new database or restore SYSTEM schemas to original state.

It's important! Don't reinstall Oracle software. Don't manually delete Oracle files. Any files, locating in Oracle home directory. Because if you do this - you won't be able to delete database and free SID, even if you restore deleted files. So you'll have additional work for uninstalling Oracle database and it's not the trivial thing.

You should go to the Database Configuration Assistant (as I work in Windows Server 2012) and select option Delete database. With next step you should choose SID of database you want to delete and specify login and password of the sysdba user. When you press Finish button your database will be deleted.

If you've delete manually files of the database or Oracle DBMS, you could recieve some errors about connection to database, or you'll have three steps instead of two. So, I hope you'll delete it properly.