Database Modeling and Design

216 Appendix: The Basics of SQL
create table item
(item_num numeric,
item_name char(20),
price
numeric,
weight numeric,
primary key (item_num));
create table order
(ord_num char(15),
cust_num numeric not null,
item_num numeric not null,
quantity numeric,
total_cost numeric,
primary key (ord_num),
foreign key (cust_num) references customer
on delete no action on update cascade,
foreign key (item_num) references item
on delete no action on update cascade);
SQL, while allowing the above format for primary key and foreign
key, recommends a more detailed format, shown below, for table order:
constraint pk_constr primary key (ord_num),
constraint fk_constr1 foreign key
(cust_num) references customer
(cust_num) on delete no action on update cascade,
constraint fk_constr2 foreign key
(item_num) references item (item_num)
on delete no action on update cascade);
in which pk_constr is a primary key constraint name, and fk_constr1
and fk_constr2 are foreign key constraint names. The word “constraint”
is a keyword, and the object in parentheses after the table name is the
name of the primary key in that table referenced by the foreign key.
The following constraints are common for attributes defined in the
SQL create table commands:
• Not null: a constraint that specifies that an attribute must have a
nonnull value.
• Unique: a constraint that specifies that the attribute is a candidate
key; that is, that it has a unique value for every row in the table.
Every attribute that is a candidate key must also have the constraint
not null. The constraint unique is also used as a clause to