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

Chapter 6: Constraints
How It Works
Our latest constraint works just as the last one did — the physical table definition is aware of the rules
placed on the data it is to contain. Just as it would not allow string data to be inserted into a numeric column,
now it will not allow a row to be inserted into the Orders table where the referenced employee in
charge of that order is not a valid EmployeeID. If someone attempts to add a row that doesn’t match
with an employee record, the insertion into Orders will be rejected in order to maintain the integrity of
the database.
Note that while we’ve added two foreign keys, there is still a line down at the bottom of our
sp_helpconstraint results (or under the Messages tab if you have Results in Grid selected) that
says No foreign keys reference this table. This is telling us that, while we do have foreign
keys in this table that reference other tables, there are no other tables out there that reference this table. If
you want to see the difference, just run sp_helpconstraint on the Customers or Employees tables
at this point, and you’ll see that each of these tables is now referenced by our new Orders table.
Making a Table Self-Referencing
162
What if the column you want to refer to isn’t in another table, but is actually right within the table in
which you are building the reference? Can a table be both the referencing and the referenced table? You
bet! Indeed, while this is far from the most common of situations, it is actually used with regularity.
Before we actually create this self-referencing constraint that references a required (non-nullable) field
that’s based on an identity column, it’s rather critical that we get at least one row in the table prior to the
foreign key being added. Why? Well, the problem stems from the fact that the identity value is chosen and
filled in after the foreign key has already been checked and enforced. That means that you don’t have a
value yet for that first row to reference when the check happens. The only other option here is to go ahead
and create the foreign key, but then disable it when adding the first row. We’ll learn about disabling constraints
a little later in this chapter.
OK — because this is a table that’s referencing a column based on an identity column, we need to get a
primer row into the table before we add our constraint:
INSERT INTO Employees
(
FirstName,
LastName,
Title,
SSN,
Salary,
PriorSalary,
HireDate,
ManagerEmpID,
Department
)
VALUES
(
‘Billy Bob’,
‘Boson’,