Foreign Key

A foreign key is a key used to link two tables together. This is sometimes called a referencing key.

Foreign Key is a column or a combination of columns whose values match a Primary Key in a different table. 

The relationship between 2 tables matches the Primary Key in one of the tables with a Foreign Key in the second table.

If a table has a primary key defined on any field(s), then you cannot have two records having the same value of that field(s).

Example:

Consider the structure of the two tables as follows:
CUSTOMERS table:

CREATE TABLE CUSTOMERS (

       ID   INT              NOT NULL,

       NAME VARCHAR (20)     NOT NULL,

       AGE INT              NOT NULL,

       ADDRESS CHAR (25) ,

       SALARY   DECIMAL (18, 2),       

       PRIMARY KEY (ID)

);

ORDERS table:

CREATE TABLE ORDERS (

       ID          INT        NOT NULL,

       DATE        DATETIME, 

       CUSTOMER_ID INT references CUSTOMERS(ID),

       AMOUNT     double,

       PRIMARY KEY (ID)

);

If ORDERS table has already been created, and the foreign key has not yet been set, use the syntax for specifying a foreign key by altering a table.

ALTER TABLE ORDERS 

   ADD FOREIGN KEY (Customer_ID) REFERENCES CUSTOMERS (ID);

DROP a FOREIGN KEY Constraint:

To drop a FOREIGN KEY constraint, use the following SQL:

ALTER TABLE ORDERS

   DROP FOREIGN KEY;