SQL - Foreign Key

A foreign key is a key used to link two tables together. This is sometimes called a referencing key.
Primary key field from one table and insert it into the other table where it becomes a foreign key ie. 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 can not 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, CISTOMER_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, 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;