SQL - FOREIGN KEY

Foreign Keys and referential constraints allow to set relationships between tables and modify some of the database engine’s actions. A foreign key is a column that uniquely references a record in another table.

For Example:
Create a table named 'EMPLOYEE' and set relationship with the table 'DEPARTMENT'.

mysql> create table employee (emp_id int(3) primary key,
    -> emp_name varchar(30) not null,
    -> desig varchar(20),
    -> dept_code varchar(3),
    -> date_join date,
    -> basic_pay decimal(10,2),
    -> da decimal(10,2),
    -> gross_pay decimal(10,2),
    -> foreign key(dept_code) 
    -> references department(dept_id));
Query OK, 0 rows affected (0.71 sec)

mysql>

To see the structure:
mysql> desc employee;
+-----------+---------------+------+-----+---------+-------+
| Field     | Type          | Null | Key | Default | Extra |
+-----------+---------------+------+-----+---------+-------+
| emp_id    | int(3)        | NO   | PRI | NULL    |       |
| emp_name  | varchar(30)   | NO   |     | NULL    |       |
| desig     | varchar(20)   | YES  |     | NULL    |       |
| dept_code | varchar(3)    | YES  | MUL | NULL    |       |
| date_join | date          | YES  |     | NULL    |       |
| basic_pay | decimal(10,2) | YES  |     | NULL    |       |
| da        | decimal(10,2) | YES  |     | NULL    |       |
| gross_pay | decimal(10,2) | YES  |     | NULL    |       |
+-----------+---------------+------+-----+---------+-------+
8 rows in set (0.00 sec)

mysql>