What I have to learn for a good App (inkluding Website) consisting of Databases and a central server for synchronizing?

Johannes;

I would start by working with things in chunks. Learn and build your iOS application and utilize an already setup backend API. Something like parse.com might suffice for the initial projects. After you get the application ironed out, worry about/work on migrating to your own storage server and code.

At least that is the way in which I would approach your project.

Happy coding,
Ken

Johannes,

Skills sets to acquire:

Online database querying using SQL (Structured Query Language), which

comes in different 'varieties' like MySQL and PostGRESQL

POST-ing and GET-ing data from a web page (to have a query to submit)

POST and GET are HTTP "request methods":

https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol#Request_methods

Under Swift you can also use what is called a 'NSURLConnection':

http://www.brianjcoleman.com/tutorial-post-to-web-server-api-in-swift-using-nsurlconnection/

Notice on the page linked to above that there sits a php script.

This is called using a REST-ful API.

REST stands for "representational state transfer":

https://en.wikipedia.org/wiki/Representational_state_transfer

The php script "catch" your data request and translates it into a SQL query.

Then catches the SQL query result and 'throw" them back to your Swift iOS to catch and use.

You will probably want to cache the remote database locally for when the mobile iOS device is not connected to the internet and re-sync the local cache as a refresh from the remote serve becomes available.

I find frequently that Ken often gives rather incomplete answers (as in the answer he gave above).

However if you want to find out more about using parse (and using a 'backend API') with Swift

then here's a few links to start:

http://www.raywenderlich.com/15916/how-to-synchronize-core-data-with-a-web-service-part-1

http://www.raywenderlich.com/17927/how-to-synchronize-core-data-with-a-web-service-part-2