Any conlumn in the unique constraint must not have null value, otherwise the update will violate the constraint and cause duplicates. This is considered as a bug #8173 of MySQL with long history, ...

There are so many problems with foreign key constraints that we don't know where to start: Foreign key constraints make life very complicated, because the foreign key definitions must be stored in a ...

Exception message: Cannot add or update a child row: a foreign key constraint fails (`HMEBooking`.`Invoice`, CONSTRAINT `Invoice_ibfk_7` FOREIGN KEY (`BookingId`) REFERENCES `Booking` (`Id`) ON DELETE ...