In order to use Oracle REST Database Services (ORDS), do we need to purchase a separate license or is it covered by a Oracle Database license?

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There is no separate licensing model for Oracle REST Database Services. According to Oracle, ORDS “is included […] with any Oracle Database license”, even in the Express edition.

Source: official Oracle FAQ Website (http://www.oracle.com/technetwork/developer-tools/rest-data-services/learnmore/faq-3561782.html#three), information retrieved 2018-02-05