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What is the best practice when deleting records from a database?
For example, if you have got this online booking tours application.
You have a tour table with a record to be removed, but the tour table also links to the booking table and customer table. Now should the record be allowed to be removed, or should there be a field like a flag that say archive, etc.
What’s best practices?
Regards,
Tea.
As Branko said, it depends on your requirements (and your database layout).
If you want to keep a full history, you probably wouldn’t be deleting much. Just setting a cancelled flag would probably be best.
Just in terms of your example database design, I’d say tour should be linked to booking, but not to customer (booking should provide a many-to-many mapping of tour to customer).
To build on your example, you probably shouldn’t be allowed to delete a customer that is booked on any tour (or the booking needs to also be deleted). When a tour is deleted, all bookings for that tour needs to be deleted (and you possibly need triggers on deletion of a booking (or a tour) to notify the customers that their bookings have been cancelled, or this trigger can be handled outside of the database).
As you can probably see from the above, there are no concrete rules, you need to logically think about what should or shouldn’t be allowed for your specific database design and what needs to happen when something is deleted.
The default is ON DELETE NO ACTION for a foreign key, which is the safest setting. It should go without saying that you should not freely use ON DELETE CASCADE or some non-default setting across the board without specifically having thought about each instance of that setting and whether using that setting is justified.
You also always ideally need to design your database in such a way that, if someone were to take it over, it would be difficult to accidentally delete the wrong things because this person doesn’t know the exact tricks of using the database (e.g. which things need to be done in what order). A more strict or safer setting for your foreign keys would make this happening more difficult.