Foreign Key:

A foreign key is a field (or fields) that points to the primary key of another table. The purpose of the foreign key is to ensure referential integrity of the data. In other words, only values that are supposed to appear in the database are permitted.

Below are some examples of how to specify the foreign key when creating the ORDERS table:

MySQL:

CREATE TABLE ORDERS
(Order_ID integer,
Order_Date date,
Customer_SID integer,
Amount double,
Primary Key (Order_ID),
Foreign Key (Customer_SID) references CUSTOMER(SID));

Oracle:

CREATE TABLE ORDERS
(Order_ID integer primary key,
Order_Date date,
Customer_SID integer references CUSTOMER(SID),
Amount double);

SQL Server:

CREATE TABLE ORDERS
(Order_ID integer primary key,
Order_Date datetime,
Customer_SID integer references CUSTOMER(SID),
Amount double);