> NoSQL came into existence because the databases at the time couldn’t handle the scale required. Arguably the first "NoSQL" database was a DIT, generally accessed via the LDAP protocol. OpenLDAP is one of the better known open source instances, but Novell eDirectory was there in the early part of this century, as was MS AD. In the case of Directory Services, it's a completely different approach to both SQL and the c…
There were NoSQL databases well before LDAP was a thing. We called them "Object databases". The idea was that doing object relational mapping was a PITA and wouldn't it be better to store your domain objects directly in the database? (As an aside, in case you are curious, the answer is: No. Generally it's not a good idea) Of course if your data is not relational, you can't do SQL. :-)
The relational model is, as I understand it, fully general, so this literally cannot be the case. There is a problem of “if you do not know the shape of the data in advance” and “if the RDBMSs available to you do not efficiently handle data of the particular shape and access pattern you are using”, but “relational” isn't an inherent property of data, it's a property of a data model, and any data that can be modelled can be modelled relationally. That doesn't mean it is efficiently handled by every, or any, existing RDBMS, though.