REBOL/Command, Rebol Technologies enterprise product, is the only version that comes with database abilities out of the box. This includes support for Oracle, ODBC, and MySql. It is the stated intention of Rebol Technologies to release the source code for the database protocols, but in the meantime third party developers have written interfaces for the free REBOL/Core to some of the more popular database products.
In many instances, the database adaptor has been written as a REBOL protocol, and access to the database is via a virtual port. Database queries are then inserted ( sent ) into the port, and results are retrieved by reading from the port.
MySQL database interface - a scheme for /Core that will allow to connect to a MySQL database server, send queries and retrieve resulting recordsets. This driver is 100% written in REBOL and doesn't require any additional library (like libmysql). Open-source : BSD-like license.
PostgreSQL interface - a scheme for /Core that will allow you to connect to a PostgreSQL database server, send queries and retrieve resulting recordsets. This driver is 100% written in REBOL and doesn't require any additional library. It works with all REBOL products (/View, /Pro, /Command, /Link,...) and on all platforms supported by REBOL ! You only need one file to make it work and its size is less 20Kb! Open-source : BSD-like license.
RebDB is a small but highly efficient pseudo-relational DBMS written entirely in REBOL/Base syntax, meaning that it will run on any platform that REBOL/Core runs on. Open-source : BSD-like license.
SQL-Protocol is also a small but efficient RDBMS written entirely in REBOL and run on any platform that REBOL/Core runs on. SQL-Protocol use an SQL like dialect and allows you to make JOINs in your query. Moreover, SQL-Protocol can use text databases compatible with the Microsoft OBDC text driver and allows you to share data between the Rebol and the Microsoft world. License: public domain.
SQLite database interface - a simplified wrapper to the SQLite library. Usable in freeware interpreter since version 2.7.6. Open source: BSD-like license.