Category: Constraints

Kenneth Fisher discusses foreign key constraints:

I have to be certain to delete from OrderDetail first, then Order (to maintain the RI) and then load Order first then OrderDetail. No big deal in this simple example, but what if I’m dealing with a dozen tables? Or I’m only re-loading the parent (Order)?

The easiest thing to do is to disable the foreign key, load your data, and then re-enable the foreign key. You might be tempted to skip that last step but don’t. RI is very important and in fact a trusted foreign key can be used by the optimizer to improve your query plan. It’s easy to say that the application doesn’t have bugs that cause problems with referential integrity. It’s not like you’re ever going insert an OrderDetail without an Order right? Unfortunately it’s far to easy for mistakes to happen. Maybe not in the application, it might be a mistake in an update meant to fix something else. Our job is to protect the data, and RI is an important part of that. So by all means disable a foreign key to help with a load but make sure you turn it back on when you are done.

If a foreign key constraint isn’t trusted, the optimizer won’t be able to assume relational integrity, and so it’s possible that the optimizer could make sub-par choices when joining tables with a foreign key constraint.

Shane O’Neill discusses primary key and foreign key constraints:

If you check the two definitions for Primary key and Foreign key you’ll see that, even though they are both called keys, they serve two different purposes; namely identifying rows and enforcing links.

And those two purposes are not mutually exclusive!

A column/column combo that identifies a row can also be used to enforce a link back to another table (or itself, as shown above with Foreign keys).

The assumption, that if you were one then you couldn’t be the other, was incorrect. If your business rules call for it, don’t let a column being one type of key stop it from being the other.

Not included but still important are unique constraints.

James Anderson shows that delete statements will ignore check constraints:

I know it sounds a bit odd but DELETE statements really do ignore table constraints. Running the code below in a test database will setup a test to prove this.

James gives a couple pieces of advice on how to implement this scenario if you find yourself needing to do something like this.