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

How It Works
Once you have successfully run the preceding code, run sp_help, and you should see your new constraint
reported under the constraints section of the sp_help information. If you want to get even more to the
point, you can run sp_helpconstraint. The syntax is easy:
EXEC sp_helpconstraint
Run sp_helpconstraint on our new Orders table, and you’ll get information back giving you the
names, criteria, and status for all the constraints on the table. At this point, our Orders table has one
FOREIGN KEY constraint and one PRIMARY KEY constraint.
Our new foreign key has been referenced in the physical definition of our table, and is now an integral
part of our table. As we discussed in Chapter 1, the database is in charge of its own integrity. Our foreign
key enforces one constraint on our data and makes sure our database integrity remains intact.
Unlike primary keys, foreign keys are not limited to just one per table. We can have between 0 and 253
foreign keys in each table. The only limitation is that a given column can reference only one foreign key.
However, you can have more than one column participate in a single foreign key. A given column that is
the target of a reference by a foreign key can also be referenced by many tables.
Adding a Foreign Key to an Existing Table
Just like with primary keys, or any constraint for that matter, there are situations where we want to add
our foreign key to a table that already exists. This process is similar to creating a primary key.
Try It Out Adding a Foreign Key to an Existing Table
Let’s add another foreign key to our Orders table to restrict the EmployeeID field (which is intended to
have the ID of the employee who entered the order) to valid employees as defined in the Employees
table. To do this, we need to be able to uniquely identify a target record in the referenced table. As I’ve
already mentioned, you can do this by referencing either a primary key or a column with a UNIQUE constraint.
In this case, we’ll make use of the existing primary key that we placed on the Employees table
earlier in the chapter:
ALTER TABLE Orders
ADD CONSTRAINT FK_EmployeeCreatesOrder
FOREIGN KEY (EmployeeID) REFERENCES Employees(EmployeeID);
Chapter 6: Constraints
When you run sp_helpconstraint on this table, the word “clustered” will appear
right after the reporting of the PRIMARY KEY. This just means it has a clustered index.
We will explore the meaning of this further in Chapter 9.
Now execute sp_helpconstraint again against the Orders table, and you’ll see that our new constraint
has been added.
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