The use command lets you access an existing database if you are a known user:
use database_name
For example, to access the pubs2 database, enter:
use pubs2
It is likely that you are automatically connected to the master database when you log in to SAP ASE, so to use another database, issue the use command. Use alter login to specify the default database for a login. Only a system administrator can change the default database for another user.