If you modify the current DEFAULT DATABASE, the default database is changed for later user sessions. You can submit a DATABASE request to change the default database for the current session (see DATABASE for information about the DATABASE statement).
The default database for a user cannot be set to NULL, though you can specify the name of a database that does not exist. Note that if you do this, the system aborts the request and returns an error when the user attempts to create or reference an object within a database that does not exist.
To remove the current DEFAULT DATABASE, you can set it to the user name associated with the MODIFY USER request. This is the default setting if a DEFAULT DATABASE clause has not been specified.