For those who need or prefer directly dumping all their contacts from an OwnCloud database, below are commands to do it from a PostgreSQL installation.
Remember to put your OwnCloud user-name in place of the “…” in the query.
psql owncloud --tuples-only --no-align -c \
"SELECT encode(carddata, 'escape') FROM oc_cards WHERE addressbookid = (SELECT id FROM oc_addressbooks WHERE principaluri = 'principals/users/...')" \
If you happen to have accented, or other non-ASCII, characters then you may need to convert from octal (“\nnn”) to UTF before trying to import elsewhere.