问题
Maybe this is sort of a naive question...but I think that we should always have cascading deletes and updates. But I wanted to know are there problems with it and when should we should not do it? I really can't think of a case right now where you would not want to do an cascade delete but I am sure there is one...but what about updates should they be done always?
So can anyone please list out the pros and cons of cascading deletes and updates ? Thanks.
回答1:
Pros:
- When you delete a row from the Parent table all the foreign key rows are deleted
- This is usually faster than implementing this with triggers
- Orphaned rows are unlikely
Cons
- Orphans are possible
- If by mistake you delete a row in the parent table all the rows in the corresponding child tables will be deleted and it will be PITA to figure out what you deleted
回答2:
This depends on the entities that are contained in the tables: if the side of the foreign key cannot exist without the side of the primary key, it makes sense to have cascaded delete.
E. g.: An invoice line item does not have any right to survive if the invoice is deleted.
But if you have a foreign key used for the relationship "works for" for the relationship between an employee and his/her boss, would you want to delete the employee if the boss leaves the company?
In addition: a technical issue is that some ORM (object relational mapping) tools are confused if dependent table entries change without them being responsible for that.
回答3:
Pros:
- Data integrity - Can help avoid situations where a record refers to something that is no longer there.
Cons:
- Performance - Cascading deletes/updates can be sloooooooooooooooooooow.
- Complexity - It seems most people I work with are not used to cascades, so when you give them a new project that has it, they're a bit surprised the first time they trigger one of these cascades.
- As others have already mentioned, can really mess things up when used improperly.
回答4:
Pro: it allows you to reduce the quantity of SQL statements needed to perform delete actions.
Con: you could delete data that somewhat may be important for auditing in a later time. So it's important to keep it even if the parent row has been deleted. In this case the foreign key should receive a NULL value for example.
来源：https://stackoverflow.com/questions/3578975/what-are-the-pros-and-cons-of-cascading-delete-and-updates