MySQL - World's most popular open source database

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The MySQL database is the world's most popular open source database because of its consistent fast performance, high reliability and ease of use. It is used by individual Web developers as well as many of the world's largest and fastest-growing organizations to save time and money powering their high-volume Web sites.

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rqlite - The lightweight, distributed relational database built on SQLite.

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rqlite is a distributed relational database, which uses SQLite as its storage engine. rqlite uses Raft to achieve consensus across all the instances of the SQLite databases, ensuring that every change made to the system is made to a quorum of SQLite databases, or none at all. It also gracefully handles leader elections, and tolerates failures of machines, including the leader. rqlite is available for Linux, OSX, and Microsoft Windows.rqlite gives you the functionality of a rock solid, fault-tolerant, replicated relational database, but with very easy installation, deployment, and operation. With it you've got a lightweight and reliable distributed relational data store. Think etcd or Consul, but with relational data modelling also available.

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PostgreSQL - Powerful, Open Source Object-Relational Database System

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PostgreSQL is a powerful, open source object-relational database system. It has more than 15 years of active development and a proven architecture that has earned it a strong reputation for reliability, data integrity, and correctness. It is fully ACID compliant, has full support for foreign keys, joins, views, triggers, and stored procedures (in multiple languages). It has native programming interfaces for most of the programming language.

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HyperSQL - Java Database

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HyperSQL supports the widest range of SQL Standard features seen in any open source database engine. It offers a small, fast database engine which offers in-memory and disk-based tables and supports embedded and server modes. It is a relational database engine written in Java, with a JDBC driver, conforming to ANSI SQL:2008. It alsi supports transaction isolation, multiversion concurrency and ACID, Java stored procedures, Scalability and lot more.

database relational persistence relational-database in-memory-database embedded-database

Apache Derby - Relational Database implemented in Java

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Apache Derby is an open source relational database implemented entirely in Java. It has a small footprint that makes it easy to embed in any Java-based application, but it also supports the more familiar client/server mode. It is based on the Java, JDBC, and SQL standards, making code developed more portable to standards-compliant databases.

database embedded-database in-memory relational-database

atomspace - The OpenCog hypergraph database, query system and rule engine

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The OpenCog AtomSpace is a knowledge representation (KR) database and the associated query/reasoning engine to fetch and manipulate that data, and perform reasoning on it. Data is represented in the form of graphs, and more generally, as hypergraphs; thus the AtomSpace is a kind of graph database, the query engine is a general graph re-writing system, and the rule-engine is a generalized rule-driven inferencing system. The vertices and edges of a graph, known as "Atoms", are used to represent not only "data", but also "procedures"; thus, many graphs are executable programs as well as data structures. The AtomSpace is a platform for building Artificial General Intelligence (AGI) systems. It provides the central knowledge representation component for OpenCog. As such, it is a fairly mature component, on which a lot of other systems are built, and which depend on it for stable, correct operation in a day-to-day production environment.

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express-starter - It's a hackathon-starter fork, but designed to use PostgreSQL by default (or MySQL)

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A boilerplate for Node.js web applications, which use RDBS (relational database system) with prefered PostgreSQL instead of overhyped MongoDB. Project based on and inspired by Hackathon Starter.

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