I am developing a website that will manage data for multiple entities. No data is shared between entities, but they may be owned by the same customer. A customer may want to manage all their entities from a single "dashboard". So should I have one database for everything, or keep the data seperated into individual databases? Is there a best-practice? What are the positives/negatives for having a:

• database for the entire site (entity has a "customerID", data has "entityID")
• database for each customer (data has "entityID")
• database for each entity (relation of database to customer is outside of database)

Multiple databases seems like it would have better performance (fewer rows and joins) but may eventually become a maintenance nightmare.