You will see all processes on the Host page by choosing "All processes". Databases are typically monitored from the application by tracing calls to the database services. If you install OneAgent on a DB host, you get additional metrics and monitoring about the operating system, the DB processes, etc. For deeper database monitoring, you will need to provide credentials to connect to the DB and OneAgent will provide also metrics from the database.
"No processes detected" only means, there are no detected processes for deep monitoring (Java/NET/Go/....). For database hosts you typically don't need full stack agent, you can switch the agent into cloud infrastructure mode. You can switch the mode on the host page, choose edit and then switch the monitoring mode.