Database Management Systems (dbms)

Terminology..

• Degree
• The number of attributes of a relation.
• Relation schema
• A relation name followed by set of attributes
• A relation schema is denoted by

R (A1, A2, A3,….,An)

R – Relation name

A1, A2, A3,….,An - – list of attributes

Terminology..

• Cardinality
• The cardinality of the relation is the number of tuples present in the body, which varies with time.

Example - Terminology

St – No	Name	Address	Dob
S 1	Kamal	Matara	01-05-1980
S 2	Himali	Kaluthara	31-10-1981
S 3	Renuka	Galle	19-06-1979

STUDENT
Tuple
R (Student)
Degree of student – 4
Cardinality of student -3
R
Attribute

Properties of Relation

• The relation has a name that is distinct from all other relations
• Tuple of relation do not have any particular order.
• Each attribute has a distinct name
• Attributes are unordered.
• There are no duplicate tuple.
• All attribute values are atomic.

i.e. Not divisible into components.

Key Types of a Relation

• Candidate key
• A candidate key is an attribute or set of attributes whose values uniquely identify a tuple of a relation.
• There may be more than one candidate keys for a relation.

E.g. Consider the following relation schema Department (d#, dname, location )

• Assumption : d# and dname are unique
• Candidate keys:{d#} and {dname}