Database Quiz - lesson 1

Hi, there. Before designing good database Eg, MS Access, MS VB, MS SQL and etc. Let's figure out how good you know about meaning of database terminology. Do you know what primary key is :
Eg, IC no is primary key (uniquely identified in a table) ; name is not. Name could possibly exist more than once in the world. Got it ?

Foregin 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.

Eg. 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.

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);