Database world - Various databases flavours

Various Databases flavours:
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RDBMS
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Oracle
MySQL
MS SQL Server
PostgreSQL
DB2

5 categories of NoSQL Databases (Top DB of each category):
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Key Value Store - Redis
Wide Column Store - Cassandra
Document Store - Mongo DB
Graph - Neo4j
Search Engine - Elasticsearch

References:
http://db-engines.com/en/ranking