Keys in a relational database

1. Name: Sana Salah_ud_din
M.Sc CS (final)
Roll#: 138005
Topic: Keys In Relational
Database

2. AT A Glance:
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What is :
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A Key ?
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A Super Key ?
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A Candidate Key ?
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A Primary Key ?
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An Alternate Key ?
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3. relational Key:
 A KEY is an attribute or set
of attributes that uniquely
identifies a TUPLE in a relation.
 There are two main types of key:
 A Super Key
 and a Foreign key
Relational
Keys
Super
Key
Foreign
Key
Candidate
Key
Primary
Key
Alternate
key
Candidate
Key

4. Super Key:
 An attribute, or set of attributes, that uniquely
identifies a tuple within a relation.
 A Super key may contain additional attributes that are
not necessary for unique identification.
 A super key is the most
general type of key.
EXAMPLE:
 As shown in the figure the relat-
ion Branch has a super key

5. Candidate key:
 A key which is the candidate for being selected as a
primary key.
 A candidate key contains only the attributes which can
uniquely identify a tuple in a relation.
 As shown in the figure --- in the
relation Branch the attributes
branchNo & postcode can serve
as candidate keys because both
can uniquely identify a tuple in

6. Primary key:
 The candidate key that is selected to identify tuples
uniquely within the relation.
 Some most important points about primary key are:
 A relation can have only one primary key.
 Each value in primary key attribute must be unique.
 Primary key cannot contain null values.
 As shown in the figure----
the attribute branchNo can
serve as primary key because

7. Alternate key:
 The candidate keys that are not selected as primary
key are known as alternate keys.
 As shown in the figure
the relation Branch has
branchNo serving as pri-
mary key and postcode
serving as alternate key
because postcode can

8. Composite Key:
 A primary key that consists of two or more attributes is
known as COMPOSITE KEY.
 As shown in the figu-
re– the relation Branch
has two attributes
serving as primary key
so this is a composite
key,

9. Foreign key:
 An attribute, or set of attributes, within one relation
that matches the candidate key of some relation..
 As shown in the figur-
e in the relation Staff
The attribute branchNo
is the foreign key
because it is the prim-
ary key in the relation
Branch.

10. The end