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

Just remember that each constraint stands alone — you add, change, or delete each of them
independently.
Next, look at our foreign key declaration:
FOREIGN KEY (OrderID)
REFERENCES Orders(OrderID)
We’ve declared our OrderID as being dependent on a “foreign” column. In this case, it’s for a column
(also called OrderID) in a separate table (Orders), but as we saw earlier in the chapter, it could just as
easily have been in the same table.
Now, to get to the heart of our cascading issue, we need to look at our ON clauses:
ON UPDATE NO ACTION
ON DELETE CASCADE
We’ve defined two different referential integrity actions. As you might guess, a referential integrity action
is what you want to have happen whenever the referential integrity rule you’ve defined is invoked. For
situations where the parent record (in the Orders table) is updated, we’ve said that we do not want that
update to be cascaded to our child table (OrderDetails). For illustration purposes, however, I’ve chosen
a CASCADE for deletes.
Note that NO ACTION is the default, and so specifying this in our code is optional. While the “typical”
way of coding this is to leave out the NO ACTION, I would encourage you to include the NO ACTION
explicitly to make your intent clear.
Let’s try an insert into our OrderDetails table:
INSERT INTO OrderDetails
VALUES
(1, ‘4X4525’, ‘This is a part’, 25.00, 2);
Unless you’ve been playing around with your data some, this generates an error:
Chapter 6: Constraints
There is something of a “gotcha” when creating foreign keys that reference the same
table the foreign key is being defined on. Foreign keys of this nature are not allowed
to have declarative CASCADE actions. The reason for this restriction is to avoid
cyclical updates or deletes; that is, situations where the first update causes another,
which in turn tries to update the first. The result could be a never-ending loop.
Msg 547, Level 16, State 0, Line 1
The INSERT statement conflicted with the FOREIGN KEY constraint
“FKOrderContainsDetails”. The conflict occurred in database “Accounting”, table
“dbo.Orders”, column ‘OrderID’.
The statement has been terminated.
Why? Well, we haven’t inserted anything into our Orders table yet, so how can we refer to a record in
the Orders table if there isn’t anything there?
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