> 1) Is it possible to act on data contained in physical files through
> JDBC connector? (SELECT, INSERT INTO,...) - the DB2 should present them
> as SQL tables;
yes, physical (and logical ) files are seen as Tables by DB2.
> 2) The relations (keys) are contained in logical files. When accessing
> via JDBC/SQL, do I have to pay attention to relationships in logical
> files, or are elements like referential integrity safeguarded by the RDBMS?
I do not know much about referential integrity. As far as logicals, logical files are found in SYSTABLES. Meaning, you can insert into, update, delete from a logical file.
> 3) Best practice: Should I ask Stored Procedures to call via JDBC or act
> directly on the data?
yes, access the database thru sql procedures. One advantage is the sql procedure can adopt the authority of the owner of the procedure. This way, *public rights to the database table can be restricted.
> 4) If I download the DB2 free edition, could I test this DDS/SQL
> interaction behaviour?
the free version is DB2 for windows or linux? On the IBM i DB2 is not installed separately. It is integrated into the system. You need IBM i hardware and OS to use SQL on the system.