Is it impossible to have the inverse? (That is, multiple databases, single instance.) This worries me because in our company we have several databases, most of them not actively used. I wonder if it's possible for me to have one instance where several of my databases could attach.
On the surface, it appears as though the inverse is not true. It appears as though you cannot have multiple databases running under one instance. Without OPS or RAC, you must have one instance for each database if you want to access data in that database. However, it is possible to access another database's data through an instance by using Database Links. With DB Links, you can connect to remote instances without signing off of your current instance. In this manner, you can access data in multiple instances all from the same Oracle session!