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>Sometimes when I create the View in the View-Designer from the Database-Designer I'm able to create the View and running the Query but later when I open the View for editing the View-Designer states that the View is invalid and the different joining-keys (from MainTbl and ChildTbl) doesn't exists.

The most probable reason here is that View Designer does not support parallel joins from one table to 2 or more others. It shows "Column someColumn is not found" error in this case.
You can create those views programmatically.

Nick