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

‘Head Cook & Bottle Washer’,
‘123-45-6789’,
100000,
80000,
‘1990-01-01’,
1,
‘Cooking and Bottling’
);
Now that we have a primer row, we can add in our foreign key. In an ALTER situation, this works just the
same as any other foreign key definition. We can now try this out:
ALTER TABLE Employees
ADD CONSTRAINT FK_EmployeeHasManager
FOREIGN KEY (ManagerEmpID) REFERENCES Employees(EmployeeID);
There is one difference with a CREATE statement. You can (but you don’t have to) leave out the FOREIGN KEY
phrasing and just use the REFERENCES clause. We already have our Employees table set up at this point,
but if we were creating it from scratch, the script would appear as follows (pay particular attention to
the foreign key on the ManagerEmpID column):
CREATE TABLE Employees (
EmployeeID int IDENTITY NOT NULL
PRIMARY KEY,
FirstName varchar (25) NOT NULL,
MiddleInitial char (1) NULL,
LastName varchar (25) NOT NULL,
Title varchar (25) NOT NULL,
SSN varchar (11) NOT NULL,
Salary money NOT NULL,
PriorSalary money NOT NULL,
LastRaise AS Salary -PriorSalary,
HireDate smalldatetime NOT NULL,
TerminationDate smalldatetime NULL,
ManagerEmpID int NOT NULL
REFERENCES Employees(EmployeeID),
Department varchar (25) NOT NULL
);
Chapter 6: Constraints
It’s worth noting that, if you try to DROP the Employees table at this point (to run the
second example), you’re going to get an error. Why? Well, when we established the
reference in our Orders table to the Employees table, the two tables became “schemabound;”
that is, the Employees table now knows that it has what is called a dependency
on it. SQL Server will not let you drop a table that is referenced by another table. You
have to drop the foreign key in the Orders table before SQL Server will allow you to
delete the Employees table (or the Customers table for that matter).
In addition, doing the self-referencing foreign key in the constraint doesn’t allow us
to get our primer row in, so it’s important that you do it this way only when the column
the foreign key constraint is placed on allows NULLs. That way the first row can
have a NULL in that column and avoid the need for a primer row.
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