FOREIGN KEY Constraint

• 
• Defined at either the table level or the column level
• CREATE TABLE emp(employee_id numeric(6), last_name varchar(25) NOT NULL, email varchar(25), salary numeric(8,2), commission_pct numeric(2,2), hire_date DATE NOT NULL, ... department_id numeric(4), CONSTRAINT emp_dept_fk FOREIGN KEY (department_id) REFERENCES dept(deptno), CONSTRAINT emp_email_uk UNIQUE(email));

FOREIGN KEY Constraint - Keywords :

• FOREIGN KEY : Defines the column in the child table at the table-constraint level
• REFERENCES : Identifies the table and column in the parent table
• ON DELETE CASCADE : Deletes the dependent rows in the child table when a row in the parent table is deleted
  
• ON DELETE SET NULL : Converts dependent foreign key values to null