Database Reference
In-Depth Information
tifies the table's subject or the one that is the most meaningful to everyone in the organiza-
tion. For example, consider the Student table again in following figure.
In this table we have two keys; one is Registration No that is
candidate key
(CK) and an-
other is combination of Name and Address fields called
composite candidate key
(CCK).
Either of the candidate keys (Registration No or Name + Address) within the table could
serve as the primary key. Most of the users prefer to choose Registration No if everyone in
the college is accustomed to using this number as a means of identifying students. Selected
candidate key becomes the primary key of the table and is managed by the Elements of a
Primary Key. These elements are exactly the same as those for the candidate key.
4.8.3 Rules for Establishing a Primary Key (PK)
Each table must have one and only one primary key. Only one primary key is necessary
for a particular table.
Each primary key within the database must be unique. No two tables should have the
same primary key unless one of them is a subset table. A table throughout the database
