SQL 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.
 
Table CUSTOMER

column name	characteristic
SID	Primary Key
Last_Name
First_Name

Table ORDERS

column name	characteristic
Order_ID	Primary Key
Order_Date
Customer_SID	Foreign Key
Amount

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

if you alter the reference:

ALTER TABLE ORDERS
ADD FOREIGN KEY (customer_sid) REFERENCES CUSTOMER(SID);