Adding a FOREIGN KEY Constraint

SQL> CREATE TABLE departments
  2  (department_id             number(10)        primary key,
  3   department_name           varchar2(50)      not null
  4  );

Table created.

SQL>
SQL> CREATE TABLE employees
  2  ( employee_id          number(10)      not null,
  3    last_name            varchar2(50)    not null,
  4    job_id               varchar2(30),
  5    salary               number(6),
  6    manager_id           number(6)
  7  );

Table created.

SQL>
SQL>
SQL> ALTER TABLE employees
  2  ADD CONSTRAINT by_fk
  3  department_id REFERENCES departments(department_id);

Table altered.

SQL>