You can even join together data from Hive, Cassandra, MySQL, PostgreSQL, Kafka, etc., all in one query. We don't have a connector for Mongo yet, but contributions are welcome!

The Cassandra connector was actually an external contribution. We don't use it at Facebook.

SQL is not part of the Cassandra or Mongo feature sets. They have certain analytic possibilities, but not if you want to use SQL (window functions for example), or most of the BI client tools associated with data warehousing.

There's also Shard-Query and (not free) Amazon Redshift.

Or you could just shard your regular community Postgre or MySQL.