Or user group... or active directory group... or admin group. super user. etc. Or... you can just split things by tables. Or even shard by databases where I do

Or user group... or active directory group... or admin group. super user. etc. Or... you can just split things by tables. Or even shard by databases where I don't have access to your database and vice versa. doing stuff in the application and leaving everything in one database/schema is an option... but don't think you aren't making trade offs and leaving open possible issues by not taking the more comprehensive option like sharding. And that's just one question to ask. Another is what about upgrading the database and segregating customers. can't do that if everyone is on the same database/schema. What if a customer doesn't want to be updated or upgraded? Much like companies paying for Windows XP support because stuff they have relies on the older version of software? "where user.id = 123" is a simple solution that quickly becomes more complicate to put it mildly.