NoSQL DBs became popular because performant distributed joins are hard.<p>Joins (and aggregate, group by, etc.) were thrown out to make horizontal scale out simpler.  Cassandra CQL, for example, shares keywords with SQL, but the key-value(ish) data model couldn&#x27;t be more different, and is much easier to scale horizontally.<p>Various systems are trying to solve the distributed join problem while keeping easy horizontal scaling, and if they succeed the NoSQL label won&#x27;t make sense for those systems any more.