Although little experienced I understand that one can give users different rights of access to the database and tables within. Asume I maintain an application for different departments with equal functionality and their own users. Do I need to use different databases for this or can I arrange it such that rights are granted on different sets of data? Each set distinct and inaccessible for eachother.

It would be the otherway around to ask if I 'have my own' database when I use an internet service for my financial administration or HRM-data. Or is data from several clients, combined in one database.

What is the key insight here? And . . . is there difference in for example SQL Server, Oracle, MySQL, etc.?

Thanks in advance,