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I believe you can choose the kind of store you want for Core Data. (SQL is probably the most popular though.)

You can choose between:

1. XML-based (the entire database is one xml file)

2. SQLite-based

3. Roll your own (by providing an implemention)

#1 is terribly slow for most cases and not publicly available on iOS.

#3 was released early in the ioS lifecycle (iOS 3 or iOS 5, depending which piece we're talking about)

SQL is, for most purposes, the only implementation, except for iTunes which used #1 for a decade after #2 clearly won.

iTunes uses a proprietary database format IIRC, but keeps a copy of it in XML on the side so other people can read it if they want.