Tables and Constraints - Pro Oracle Database 12c Administration - page 197

For reference, here's how the parent table DEPT table was created for these examples:

create table dept(
dept_id number primary key,
dept_desc varchar2(30));

A foreign key must reference a column in the parent table that has a primary key or a unique key defined on it.
DEPT is the parent table and has a primary key defined on DEPT_ID .
You can use several methods to create a foreign key constraint. The following example creates a foreign key
constraint on the DEPT_ID column in the EMP table:

create table emp(
emp_id number,
name varchar2(30),
dept_id constraint emp_dept_fk references dept(dept_id));

Note that the DEPT_ID data type isn't explicitly defined. The foreign key constraint derives the data type from the
referenced DEPT_ID column of the DEPT table. You can also explicitly specify the data type when you define a column
(regardless of the foreign key definition):

create table emp(
emp_id number,
name varchar2(30),
dept_id number constraint emp_dept_fk references dept(dept_id));

You can also specify the foreign key definition out of line from the column definition in the CREATE TABLE
statement:

create table emp(
emp_id number,
name varchar2(30),
dept_id number,
constraint emp_dept_fk foreign key (dept_id) references dept(dept_id)
);

And, you can alter an existing table to add a foreign key constraint:

alter table emp
add constraint emp_dept_fk foreign key (dept_id)
references dept(dept_id);

■ Unlike with primary key and unique key constraints, Oracle doesn't automatically add an index to foreign key
columns; you must explicitly create indexes on them. see Chapter 8 for a discussion on why it's important to create
indexes on foreign key columns and how to detect foreign key columns that don't have associated indexes.

Note