Beginning SQL

In the preceding code, CustomerId is the primary key. Alternatively, more than one column can act as
the primary key, in which case you need to add the constraint at the end of the table, after the column
definitions.
The following Try It Out shows you how to use the PRIMARY KEY constraint.
Try It Out Establishing a PRIMARY KEY Constraint
1. Execute the following SQL to create the MoreHolidayBookings table:
CREATE TABLE MoreHolidayBookings
(
CustomerId int NOT NULL,
BookingId int NOT NULL,
Destination varchar(50),
CONSTRAINT booking_pk PRIMARY KEY (CustomerId, BookingId)
);
2. Now try executing the following SQL to insert data into the MoreHolidayBookings table:
INSERT INTO MoreHolidayBookings (CustomerId, BookingId, Destination)
VALUES (1,1,’Hawaii’);
INSERT INTO MoreHolidayBookings (CustomerId, BookingId, Destination)
VALUES (1,2,’Canada’);
INSERT INTO MoreHolidayBookings (CustomerId, BookingId, Destination)
VALUES (2,2,’England’);
INSERT INTO MoreHolidayBookings (CustomerId, BookingId, Destination)
VALUES (1,1,’New Zealand’);
INSERT INTO MoreHolidayBookings (CustomerId, Destination)
VALUES (3,’Mexico’);
How It Works
Advanced Database Design
In Step 1, the booking_pk constraint is a PRIMARY KEY constraint, and the columns CustomerId and
BookingId are the primary key columns. Remember that the PRIMARY KEY constraint is a combination
of the UNIQUE and NOT NULL constraints, which means that the CustomerId and BookingId columns
cannot contain NULL values. They must also be unique in combination. Thus, you can have the same
value a number of times in CustomerId as long as BookingId is different each time, and vice versa. Note
that the NOT NULL constraint following the primary key columns is strictly necessary only with DB2, but
the code works fine on the other database systems even though NOT NULL isn’t needed.
The first three INSERT INTO statements work fine, but the fourth one doesn’t because it tries to insert
the same combination of values for CustomerId and BookingId, which the first INSERT INTO statement
already inserted. Because the primary key constraint doesn’t allow duplicates, you receive an error
message.
The final INSERT INTO also fails because it doesn’t assign a value for BookingId when inserting a new
record, so BookingId is NULL. Likewise, you receive an error message because the primary key constraint
does not allow NULL values.
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