NoSQL usually stores a “document at a time” database – More or less. Cassandra, Mongo, couchbase for example.
It is hard to store SQL relational data in chunked up pieces and retrieve it quickly add on top of that the fact that SAFE is trying to eliminate the server - there isn’t a good place for a database to be installed (Like mySQL or Oracle or Postgres etc)
Because a lot of “Big Data” (Facebook, Google Amazon etc) has their data stored on servers all over the planet the NoSQL has been gaining traction while the traditional SQL databases have been fading a bit.
SAFE will have “microservers” all over the planet - and any piece of data can be stored in SAFE as a data element – I suspect it will become a database of it’s own to one degree or another.
We will have to see how well it works and scales etc…