I've noticed that cloud managed SQL databases are pay per hour, whereas cloud managed NoSQL databases can sometimes be pay per use, with no hourly cost (so if you don't read/write any data, you don't pay anything). I have a few questions. Firstly, is this correct or is there some pay per use cloud hosted SQL databases out there? Secondly, why is this? I'm assuming it is something to do with NoSQL having no schema so data from different user's databases can use the same infrastructure and thus allow on demand use, whereas a SQL db needs it's own instance for some reason. But I am no expert so I would really appreciate if someone could explain this.
It seems like this is an extremely important distinction between SQL no NoSQL. In comparisons people tend to mostly focus on the data structure and schema, but to me these different pricing models are very significant.