Beginning Microsoft SQL Server 2008 ... - S3 Tech Training

Those Other CASCADE Actions …
So, those were examples of cascading updates and deletes, but what about the other two types of cascade
actions I mentioned? What of SET NULL and SET DEFAULT?
These were added with SQL Server 2005, so avoid them if you want backward compatibility with SQL
Server 2000, but their operation is very simple: If you perform an update that changes the parent values
for a row, then the child row will be set to either NULL or whatever the default value for that column is
(whichever you chose — SET NULL or SET DEFAULT). It’s just that simple.
Other Things to Think About with Foreign Keys
There are some other things to think about before we’re done with foreign keys. We will be coming back
to this subject over and over again throughout the book, but for now, I just want to get in a couple of
finer points:
❑ What makes values in foreign keys required versus optional
❑ How foreign keys are bi-directional
What Makes Values in Foreign Keys Required vs. Optional
The nature of a foreign key provides us two possible choices regarding how to fill in a column or
columns that have a foreign key defined for them:
❑ Fill the column in with a value that matches the corresponding column in the referenced table.
❑ Do not fill in a value at all and leave the value NULL.
Chapter 6: Constraints
You can make the foreign key completely required (limit your users to just the first option in the preceding
list) by simply defining the referencing column as NOT NULL. Since a NULL value won’t be valid in the
column and the foreign key requires any non-NULL value to have a match in the referenced table, you
know that every row will have a match in your referenced table. In other words, the reference is
required.
Allowing the referencing column to have NULLs will create the same requirement, except that the user
will also have the option of supplying no value — even if there is not a match for NULL in the referenced
table, the insert will still be allowed.
How Foreign Keys Are Bi-Directional
We touched on this some when we discussed CASCADE actions, but when defining foreign keys, I can’t
stress enough that they effectively place restrictions on both tables. Up to this point, we’ve been talking
about things in terms of the referencing table; however, once the foreign key is defined, the referenced
table must also live by a rule:
By default, you cannot delete a record or update the referenced column in a referenced
table if that record is referenced from the dependent table. If you want to be
able to delete or update such a record, then you need to set up a CASCADE action for
the delete and/or update.
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