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.
For example, say we have two tables,
a CUSTOMER table that includes all customer data,
and an ORDERS table that includes all customer orders.
The constraint here is that all orders must be associated with a
customer that is already in the CUSTOMER table.
In this case, we will place a foreign key on the ORDERS table and have
it relate to the primary key of the CUSTOMER table.
This way, we can ensure that all orders in the ORDERS table are
related to a customer in the CUSTOMER table.
In other words,
the ORDERS table cannot contain information on a customer that is not
in the CUSTOMER table.
The structure of these two tables will be as follows:
-----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
In the above example, the Customer_SID column in the ORDERS table is a
foreign key pointing to the SID column in the CUSTOMER table.
Below we show 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);
Below are examples for specifying a foreign key by altering a table.
This assumes that the ORDERS table has been created, and the foreign
key has not yet been put in:
MySQL:
ALTER TABLE ORDERS
ADD FOREIGN KEY (customer_sid) REFERENCES CUSTOMER(SID);
Oracle:
ALTER TABLE ORDERS
ADD (CONSTRAINT fk_orders1) FOREIGN KEY (customer_sid) REFERENCES CUSTOMER(SID);
SQL Server:
ALTER TABLE ORDERS
ADD FOREIGN KEY (customer_sid) REFERENCES CUSTOMER(SID);
Sumber:
http://www.1keydata.com/sql/sql-foreign-key.html