Amazon RDS instances are regular MySQL / Oracle / databases .... You can connect to it from any device or platform if you have a driver for it.
There is a C driver for Mysql (see this question How to install MySQL Connector / C on Mac OS X? ) That works on Mac OS X and iOS.
If you want to access MySQL RDS from an iOS application, you must include this driver in your binary distribution and use the functions opened by the driver from your code.
From an architectural point of view, however, I doubt this approach. There are many advantages to accessing your database through the API level (a web application located between your iOS application and the database).
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- : API
- : API