RSQL is described as 'lightweight, simplified alternative to Microsoft SQL Server'. There are six alternatives to RSQL for a variety of platforms, including Windows, Linux, Mac, BSD and Self-Hosted solutions. The best alternative is PostgreSQL, which is both free and Open Source. Other great apps like RSQL are SQLite (Free, Open Source), MySQL Community Edition (Free, Open Source), MariaDB (Free, Open Source) and Microsoft SQL Server (Freemium).
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.
SQLite is a software library that implements a self-contained, serverless, zero-configuration, transactional SQL database engine. SQLite is the most widely deployed SQL database engine in the world. The source code for SQLite is in the public domain.
MariaDB is a backward compatible, drop-in replacement of the MySQL® Database Server. It includes all major open source storage engines. The source code for MariaDB is publically available from Launchpad. Binaries and packages are also available.
SQL Server delivers on Microsoft’s Data Platform vision by helping your organization manage your data by enabling you to store data from structured, semi-structured, and unstructured documents within the database.