When/Why to use Cascading in SQL Server?

When setting up foreign keys in SQL Server, under what circumstances should you have it cascade on delete or update, and what is the reasoning behind it?

This probably applies to other databases as well.

I'm looking most of all for concrete examples of each scenario, preferably from someone who has used them successfully.

Solution 1:

Summary of what I've seen so far:

• Some people don't like cascading at all.

Cascade Delete

• Cascade Delete may make sense when the semantics of the relationship can involve an exclusive "is part of" description. For example, an OrderLine record is part of its parent order, and OrderLines will never be shared between multiple orders. If the Order were to vanish, the OrderLine should as well, and a line without an Order would be a problem.
• The canonical example for Cascade Delete is SomeObject and SomeObjectItems, where it doesn't make any sense for an items record to ever exist without a corresponding main record.
• You should not use Cascade Delete if you are preserving history or using a "soft/logical delete" where you only set a deleted bit column to 1/true.

Cascade Update

• Cascade Update may make sense when you use a real key rather than a surrogate key (identity/autoincrement column) across tables.
• The canonical example for Cascade Update is when you have a mutable foreign key, like a username that can be changed.
• You should not use Cascade Update with keys that are Identity/autoincrement columns.
• Cascade Update is best used in conjunction with a unique constraint.

When To Use Cascading

• You may want to get an extra strong confirmation back from the user before allowing an operation to cascade, but it depends on your application.
• Cascading can get you into trouble if you set up your foreign keys wrong. But you should be okay if you do that right.
• It's not wise to use cascading before you understand it thoroughly. However, it is a useful feature and therefore worth taking the time to understand.

Solution 2:

Foreign keys are the best way to ensure referential integrity of a database. Avoiding cascades due to being magic is like writing everything in assembly because you don't trust the magic behind compilers.

What is bad is the wrong use of foreign keys, like creating them backwards, for example.

Juan Manuel's example is the canonical example, if you use code there are many more chances of leaving spurious DocumentItems in the database that will come and bite you.

Cascading updates are useful, for instance, when you have references to the data by something that can change, say a primary key of a users table is the name,lastname combination. Then you want changes in that combination to propagate to wherever they are referenced.

@Aidan, That clarity you refer to comes at a high cost, the chance of leaving spurious data in your database, which is not small. To me, it's usually just lack of familiarity with the DB and inability to find which FKs are in place before working with the DB that foster that fear. Either that, or constant misuse of cascade, using it where the entities were not conceptually related, or where you have to preserve history.

Solution 3:

I never use cascading deletes.

If I want something removed from the database I want to explicitly tell the database what I want taking out.

Of course they are a function available in the database and there may be times when it is okay to use them, for example if you have an 'order' table and an 'orderItem' table you may want to clear the items when you delete an order.

I like the clarity that I get from doing it in code (or stored procedure) rather than 'magic' happening.

For the same reason I am not a fan of triggers either.

Something to notice is that if you do delete an 'order' you will get '1 row affected' report back even if the cascaded delete has removed 50 'orderItem's.

Solution 4:

I work a lot with cascading deletes.

It feels good to know whoever works against the database might never leave any unwanted data. If dependencies grow I just change the constraints in the diagramm in Management Studio and I dont have to tweak sp or dataacces.

That said, I have 1 problem with cascading deletes and thats circular references. This often leads to parts of the database that have no cascading deletes.