How foreign key can reduce redundancy in the database design

Hi,

Answer is :

Foreign Key can not reduce redundacy in DB as it supports duplicate as well as NULL values.

But it is very useful to maintain the Referencial Integrity (Foreign Key Constrints) in DB.

Thank you. (If you have any different asnwers please post it)

i think it can reduce reduncency.

suppose we have a table

table :orders
--------------------------
customer_id       order_no      customer_address
---------------------------------------------------------------
1                       O11               US
2                       O22               UK
3                       O33               IND
1                       O44               US      --> duplicate
3                       O55               IND    --> duplicate
------------------------------------------------------------------
here customers 1,3 made two orders. so the customer_address is duplicated for each order.

we can resolve this by foreign key integrity.

table : orders
-----------------------------------------
customer_id      order_no         |          here customer_id is FOREIGN KEY
-----------------------------------------
1                      O11
2                      O22
3                      O33
1                      O44
3                      O55
--------------------------------------------

table : address
----------------------------------------
customer_id        address        |         Here customer_id is PRIMARY KEY
---------------------------------------
1                         US
2                         UK
3                         IND
------------------------------------------

now we can select as

select
       o.customer_id,
       o.order_no,
       a.address
from
       orders o,
       address a
where
       o.customer_id = a.customer_id

A column included in the definition of referential integrity which would refer to a referenced key.
Referenced key is a primary key which is combination of UNIQUE+NOT NULL".