"Problem" exists relational database
> Principle using ACID (atomicity, consistency, isolation, durability) ensure data integrity;
> Ranks of standardized storage;
> Predefined structure;
> Stored data amount "small";
> Structured Query;
NoSQL databases
> NoSQL (NoSQL = Not Only SQL), that is, "not just SQL".
> NoSQL features:
> Using the set of data stored;
> Dynamic structure definition;
> Storage streamlined;
> Scalability;
> For the cloud;
NoSQL database classification
> Key (Key-Value) stored in the database: The structure of Hash table memory (simple and easy deployment)
>Tokyo Cabinet/Tyrant，Redis，Voldemort，OracleBDB，MemcacheDB。
> Column store database: Coping distributed storage of huge amounts of data, in the form of a column family to save the data.
>Cassandra，Hbase，Riak；
> Document database: inspired by the lotus Notes, a similar form of JSON storage.
>CouchDB，MongoDB，SequoiaDB；
> Graphics (Graph) database: Neo4J, FlockDB;
> Object Storage: In a similar object-oriented syntax of the language to operate the database.
> Db4o, Slope
> Xml databases: efficient storage of XML data, and support internal XML query syntax, such as XQuery, Xpath.
>Berkely DB XML，BaseX；
NoSQL databases and SQL databases are used in combination, there is no concept of who was replaced by the two sides, flexible NoSQL database used in different scenarios.