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-- trigger which is used to delete all entries of a person by personen_id-- when it is deleted in the personen tableCREATE OR REPLACE TRIGGER removePerson_tw04n133BEFORE DELETE ON personen_tw04n133FOR EACH ROW    DECLARE        is_student number;        is_staff number;BEGIN    -- look if person is a student    SELECT COUNT(*) INTO is_student FROM studenten_tw04n133 s    WHERE s.personen_id = :old.personen_id;    --look if person is in personal table    SELECT COUNT(*) INTO is_staff FROM studenten_tw04n133 s    WHERE s.personen_id = :old.personen_id;     -- deleting all entries a student can have    IF is_student > 0 THEN        DELETE FROM noten_tw04n133 WHERE personen_id = :old.personen_id;        DELETE FROM studenten_tw04n133 WHERE personen_id = :old.personen_id;    END IF;    -- deleting all entries a person of the staff can have    IF is_staff > 0 THEN        DELETE FROM studiengangsleiter_tw04n133 WHERE personen_id = :old.personen_id;        DELETE FROM unterricht_tw04n133 WHERE personen_id = :old.personen_id;        DELETE FROM fachbereich_tw04n133 WHERE personen_id = :old.personen_id;        DELETE FROM personal_tw04n133 WHERE personen_id = :old.personen_id;    END IF;END;