USE vn;
DELETE tc FROM vn.ticketCollection tc
	JOIN (
		SELECT id, COUNT(ticketFk) counter
			FROM vn.ticketCollection
			GROUP BY ticketFk
			HAVING counter > 1
	) sub ON sub.id = tc.id;

ALTER TABLE vn.ticketCollection DROP KEY ticketCollection_UN;
ALTER TABLE vn.ticketCollection ADD CONSTRAINT ticketCollection_UN UNIQUE KEY (ticketFk);
