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Do we want two customers to have the same CustomerNo? Definitely not. It makes perfect sense for a
CustomerNo to be used as an identifier for a customer. Indeed, such a system has been used for years, so
there’s really no sense in reinventing the wheel here.
To alter our CREATE TABLE statement to include a PRIMARY KEY constraint, we just add in the constraint
information right after the column(s) that we want to be part of our primary key. In this case, we
would use:
USE Accounting;
CREATE TABLE Customers
(
CustomerNo int IDENTITY NOT NULL
PRIMARY KEY,
CustomerName varchar(30) NOT NULL,
Address1 varchar(30) NOT NULL,
Address2 varchar(30) NOT NULL,
City varchar(20) NOT NULL,
State char(2) NOT NULL,
Zip varchar(10) NOT NULL,
Contact varchar(25) NOT NULL,
Phone char(15) NOT NULL,
FedIDNo varchar(9) NOT NULL,
DateInSystem smalldatetime NOT NULL
);
Since this table already exists in our Accounting database (as we left it at the end of Chapter 5), we need
to also drop the table prior to running the CREATE TABLE script (i.e., DROP TABLE Customers). Notice
that we altered one line (all we did was remove the comma) and added some code on a second line for
that column. It was easy! Again, we just added one simple keyword (OK, so it’s two words, but they
operate as one) and we now have ourselves a primary key.
Creating a Primary Key on an Existing Table
Now, what if we already have a table and we want to set the primary key? That’s also easy. We’ll do that
for our Employees table:
USE Accounting
ALTER TABLE Employees
ADD CONSTRAINT PK_EmployeeID
PRIMARY KEY (EmployeeID);
Our ALTER command tells SQL Server:
❑ That we are adding something to the table (we could also be dropping something from the table
if we so chose)
❑ What it is that we’re adding (a constraint)
❑ What we want to name the constraint (to allow us to address the constraint directly later)
❑ The type of constraint (PRIMARY KEY)
❑ The column(s) that the constraint applies to
Chapter 6: Constraints
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