Week 4 (Wednesday and Friday) - Relational Model The...

8/3/2009
1
Relational Model
The foundation of present day databases
Session 4
y
Relational model
◦ Enables us to view data logically rather
than physically
◦ Similar to the simpler file concept of data
storage
Table
A Logical View of Data
y
◦ Has advantages of structural and data
independence
◦ Resembles a file from conceptual point of
view
◦ Easier to understand than its hierarchical
and network database predecessors
Characteristics of Tables
y
Table : two-dimensional structure
composed of rows and columns
y
Contains a group of related entities Æ
an entity set
◦ The terms entity set and table are often
used interchangeably

8/3/2009
2
Characteristics of Tables
(cont.) cont.)
y
Think of a table as a persistent relation :
y
A relation whose contents can be
permanently saved for future use
Example
Keys
y
Consist of one or more attributes that
determine other attributes
y
Primary key (PK) is an attribute (or a
combination of attributes) that uniquely
identifies any given entity (row)
y
Key’s role is based on determination
◦ If you know the value of attribute A, you can
look up (determine) the value of attribute B

8/3/2009
3
Keys
(
cont.
)
y
Key attribute
◦ Any attribute that is part of a key
y
Composite key
◦ Composed of more than one attribute
y
Superkey
◦ Any key that uniquely identifies each entity
y
Candidate key
◦ A superkey without redundancies
Null Values
y
No data entry
◦ Not permitted in primary key
◦ Should be avoided in other attributes
y
Can represent
◦ An unknown attribute value
◦ A known, but missing, attribute value
◦ A “not applicable” condition
y
Can create problems in logic and when
using formulae
y
Null is not zero
Controlled Redundancy
y
Makes the relational database work
y
Tables within the database share common
attributes that enable us to link tables
together (primary and foreign keys)
Multiple occurrences of values in a table
y
are not redundant when they are required
to make the relationship work
y
(Unwanted) redundancy is unnecessary
duplication of data

8/3/2009
4
Example
Keys
(cont.) cont.)
y
Foreign key (FK)
◦ An attribute whose values match primary key
values in the related table
y
Referential integrity
◦ FK contains a value that refers to an existing
valid tuple (row) in another relation
y
Secondary key
◦ Key used strictly for data retrieval purposes
Integrity Rules

8/3/2009
5
An Illustration of Integrity Rules
Relational Database Operators
y
Relational algebra
◦