Top Free RDBMS
1. MySQL community server
MySQL was initially started as an open source project and later acquired by Oracle. Currently, we have MySQL community server which is free, open source and distributed under license GNU GPL. MySQL Enterprise edition is non-free variant and Oracle proprietary.
MySQL is a RDBMS which can run on many variants of UNIX and Linux, NetBSD, FreeBSD, Solaris, Symbian, Novell Netware, Windows and macOS.
Features: SQL, Triggers, Referential integrity, Cursors, Views, X/Open XA distributed transaction processing, two phase commit, ACID compliance, Replication Support, Shared nothing clustering, Multiple Storage Engine support, runtime dynamically loadable and unloadable storage engine, Table partitioning
You can install MySQL community server from: https://dev.mysql.com/downloads/mysql/
PostGreSQL is free and open source. Can run on MacOS, Windows, Linux, FreeBSD, OpenBSD
Features: SQL, triggers,Referential integrity, stored procedures, and automatically updatable views, materialized views, built-in binary replication, Indexes, Inheritance, Tablespaces, Large Object (Binary LOB and Text LOB) Storage, Write-ahead logging
You can install PostGreSQL from: https://www.postgresql.org/download/
Runs on Windows, MacOs, Unix and Linux variants.
Features:Supports Stored procedures, triggers, ACID transactions, Referential Integrity, Native/API, ODBC, OLE DB, JDBC drivers, Cursors, Multi Generational Architecture (MGA storage architecture)
You can install FirebirdSQL from: https://firebirdsql.org/en/firebird-3-0/
4. Apache Derby
Apache Derby is distributed under a Apache license. It is a small-footprint database occupying a few MBs.
Features:Supports In-Memory database, SQL, Procedures, Triggers, JDBC, Cursor Mechanisms and Transactions. Supports Exclusive, Shared and Update Locks.
5. HSQLDB - HyperSQL Database
HSQLDB is free and open source. HSQLDB is distributed under a BSD License as well as hsqlLicense.
Features:HSQLDB features include: Support for in-memory as well as Disk-based tables, Ability to cancel long-running statements, Supports ANSI-92 SQL statements, SYNONYMs, fully multithreaded, supports high performance, multiversion concurrency control
JDBC/HSQLDB offers relatively high performance
SQLite is under Public domain.
SQLite is not a Client-Server RDBMS and is contained within C code library. For connecting from programming languages, specific connectors can be used. For example, SQLite for Java. Chrome, Safari, Opera and Android browser allows data store using SQLite database. Some versions of Mozilla Firefox and Mozilla Thunderbird use SQLite for storing contacts, bookmarks and cookies
Features include: INSTEAD OF triggers, dynamic typing, Concurrent Database access, JSON content managing, json1 extension, new subtype interfaces
Cons include: No support for Views, no ALTER TABLE function, Lacks System Integrity
MariaDB is free and open source. Community Developed. Syntax very similar to that of MySQLDB
Many of latest popular Linux and BSD Variants have MariaDB available by Default.
Features include: Most MySQL features are available in MariaDB till version 5.5