The data types serial is not a true type, but merely a notational convenience for creating unique identifier columns (similar to the AUTO_INCREMENT property supported by some other databases). In the current implementation, specifying:
If you created Ausgabe_Id SERIAL in another table, as a primary key, then it is ok to give it the data type of INTEGER in this table.
Hint: a quicker way to assign primary keys to your SERIAL fields is as follows:
CREATE TABLE Artikel (
Artikel_ID SERIAL PRIMARY KEY,
Ausgabe_ID INTEGER NOT NULL,
Since you assigned the type SERIAL it will never be NOT NULL. Postgres will generate a primary key name for you. Just a suggestion.
You should read the postgres manual. It was very helpful for me.