Relational databases are those databases that rely on tables, columns, rows, or schemas to organize and retrieve data. A NoSQL originally refers to an unstructured query language or non-relational database that provides a structure for storing and obtaining data presented in means other than tabular form or relationships similarly used in "relational databases". Such databases had existed since the late 1990s, but did not gain the NoSQL label until the surge in popularity in the early twenty-first century brought about by the needs of Web 2.0 companies such as Yahoo, Facebook, Google, and Amazon. NoSQL databases are increasingly used in big data and real-time web applications. These are called "Not Just SQL" due to the fact that they can support SQL-like query languages. Say no to plagiarism. Get a tailor-made essay on "Why Violent Video Games Shouldn't Be Banned"? Get an original essay Why the need for NoSQL? RDBMSs work with a relational model defined by a schema, where each table is a strictly defined collection of rows and columns and it is therefore possible to establish a relationship between each row of one table and a row of another table. Relational data can be queried and manipulated using the SQL query language. But what if it is inconvenient to store data in the form of tables, or if we have other types of relationships between records and want to quickly access the data? The emerging alternative is NoSQL. In many cases, the use of RDBMS leads to problems due to fixed schemas, which make them unsuitable for changing business requirements, as schema changes are problematic and time-consuming, causing insufficient performance and latency for new requirements and capabilities limited to scale in a cost-effective manner. The data structures used, such as key-values, wide columns, charts, or documents, are different from those used by default in relational databases, making some operations faster and sometimes more flexible. The main reason not to use a SQL database is scalability, especially considering the write-heavy workloads generated by modern web applications. It is not possible to make an app like Facebook work on a simple SQL database, except through massive partitioning and sharding, which also requires significant changes to the app's logic. That's why Facebook developed Cassandra. Types of NoSQL database models and their classification with examples: Wide Column: This type of database stores data in tables with rows and columns similar to RDBMS, but the column names and formats can vary from row to row in the table. Wide-column databases group columns of related data together. For example Accumulo, Cassandra, Druid, HBase, Vertica.Document: This type of database typically stores self-describing JSON, XML, and BSON documents. For example Apache CouchDB, ArangoDB, BaseX, Clusterpoint, Couchbase, Cosmos DB, IBM Domino, MarkLogic, MongoDB, OrientDB, Qizx, RethinkDBKey-value: This type of database emphasizes simplicity and is very useful for accelerating an application to support high-speed read and write non-transactional data processing. for example Aerospike, Apache Ignite, ArangoDB, Couchbase, Dynamo, FairCom c-treeACE, FoundationDB, InfinityDB, MemcacheDB, MUMPS, Oracle NoSQL Database, OrientDB, Redis, Riak, Berkeley DB, SDBM/Flat dbmGraph files: This type of database uses the structure graph to archive, map and query relationships. They provide indexless adjacency, so that adjacent elements are linked together without using an index. For example AllegroGraph, ArangoDB, InfiniteGraph, Apache Giraph, MarkLogic, Neo4J, OrientDB,.