"In the relational model, there is an built-in and foolproof method of ensuring and enforcing business logic and rules at the database layer, for instance that a withdrawal is charged to the correct bank account, through primary keys and foreign keys. In key-value stores, this responsibility falls squarely on the application logic and many people are very uncomfortable leaving this crucial responsibility just to the application. This is one reason why relational databases will continued to be used."
"So in summary, RDBMS’s suffer from no horizontal scaling for high transaction loads (millions of read-writes), while NoSQL databases solve high transaction loads but at the cost of data integrity and joins."
"NewSQL is a class of modern RDBMS’s that seek to provide the same scalable performance of NoSQL systems for OLTP read-write workloads while still maintaining the ACID guarantees of a traditional relational database system. The disadvantages is they are not for OLAP-style queries, and they are inappropriate for databases over a few terabytes. Examples include VoltDB, NuoDB, MemSQL, SAP HANA, Splice Machine, Clustrix, and Altibase."
Relational databases vs Non-relational databases
