Most applications use a relational database such as Oracle, MySQL, or PostgreSQL; however, there is more to data storage than just SQL databases. There are
Relational databases (Oracle, MySQL, PostgreSQL, etc.)
Document stores (MongoDB, Couchbase)
Key-value stores (Redis, Volgemort)
Column stores (Cassandra)
Graph stores (Neo4j, Giraph)
Each of these technologies (and all of the implementations) works in ...