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

Once you establish a UNIQUE constraint, every value in the named columns must be unique. If you try to
update or insert a row with a value that already exists in a column with a UNIQUE constraint, SQL Server
will raise an error and reject the record.
Since there is nothing novel about this (we’ve pretty much already seen it with primary keys), let’s get
right to the code. Let’s create yet another table in our Accounting database. This time, it will be our
Shippers table:
CREATE TABLE Shippers
(
ShipperID int IDENTITY NOT NULL
PRIMARY KEY,
ShipperName varchar(30) NOT NULL,
Address varchar(30) NOT NULL,
City varchar(25) NOT NULL,
State char(2) NOT NULL,
Zip varchar(10) NOT NULL,
PhoneNo varchar(14) NOT NULL
UNIQUE
);
Now run sp_helpconstraint against the Shippers table, and verify that your Shippers table has
been created with the proper constraints.
Creating UNIQUE Constraints on Existing Tables
Again, this works pretty much the same as with primary and foreign keys. We will go ahead and create a
UNIQUE constraint on our Employees table:
ALTER TABLE Employees
ADD CONSTRAINT AK_EmployeeSSN
UNIQUE (SSN);
Chapter 6: Constraints
Unlike a primary key, a UNIQUE constraint does not automatically prevent you from
having a NULL value. Whether NULLs are allowed or not depends on how you set the
NULL option for that column in the table. Keep in mind, however, that, if you do
allow NULLs, you will be able to insert only one of them (although a NULL doesn’t
equal another NULL, they are still considered to be duplicate from the perspective of
a UNIQUE constraint).
A quick run of sp_helpconstraint verifies that our constraint was created as planned, and tells us on
which columns the constraint is active.
In case you’re wondering, the AK I used in the constraint name here is for alternate key — much like
we used PK and FK for primary and foreign keys. You will also often see a UQ or just U prefix used for
UNIQUE constraint names.
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