Database engine to work under OS X and Windows

I'm about to write an app that should work under OS X and Windows XP/Vista/7. The app will require a database engine. Can someone suggest one that can do the following:

1. Be compatible with Windows
2. Be compatible with OS X (and preferably with iOS)
3. Preferably be available by default in those operating systems, or require minimum installation.

I understand that all three items may be hard to开发者_运维问答 come by. So just curious, whatever is available?

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My favorite is Sql Lite: http://sqlite.org/

It requires no installation, and is a small binary.