Beginning SQL

Chapter 4
128
You can add more than one constraint to a table, so long as each constraint has a different name and is
separated by a comma:
CREATE TABLE AnotherTable
(
Column1 int,
Column2 varchar(20),
Column3 varchar(12),
CONSTRAINT MyUniqueConstraint UNIQUE(Column2, Column3),
CONSTRAINT AnotherConstraint UNIQUE(Column1, Column3)
);
If you’re using DB2, you would write the following statement:
CREATE TABLE AnotherTable
(
Column1 int NOT NULL,
Column2 varchar(20) NOT NULL,
Column3 varchar(12) NOT NULL,
CONSTRAINT MyUniqueConstraint UNIQUE(Column2, Column3),
CONSTRAINT AnotherConstraint UNIQUE(Column1, Column3)
);
You can delete AnotherTable from the database:
DROP TABLE AnotherTable;
So far you’ve learned how to add a UNIQUE constraint at the time of a table’s creation. However, using
the ALTER TABLE statement, you can add and remove a UNIQUE constraint after the table’s creation.
Chapter 1 discussed the ALTER TABLE statement in relation to adding and removing columns in a table.
To add a constraint, you specify which table to alter and then state that you want to ADD a constraint.
The SQL code required to define a constraint with an ALTER TABLE statement is identical to how you
create a constraint at table creation.
The following code demonstrates how to add a constraint called MyUniqueConstraint to a table called
YetAnotherTable:
CREATE TABLE YetAnotherTable
(
Column1 int,
Column2 varchar(20),
Column3 varchar(12)
);
ALTER TABLE YetAnotherTable
ADD CONSTRAINT MyUniqueConstraint UNIQUE(Column2, Column3);