Revised by Ivor Perry

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The Relational Model
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The Relational Model

• Proposed by Codd in 1970 - Has a mathematical
  basis the term relation comes from this
• Determining data structure.
• Data is stored in a structure of relations
  (tables) defined by a data definition language
  (DDL).
• The elements of data structure used in relational
  models are relations (tables), attributes, tuples
  (rows), and domains.
• Defining data integrity.
• Data integrity means that data remains stable,
  secure, and accurate.
• It is maintained by internal constraints known as
  integrity rules that are invisible to users.

Curtis Cobham, Business Information Systems,
Prentice Hall Ch8
3
The Data Model

• 3 Components

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Manipulating Data

• Relations can be manipulated and changed using a
  data manipulation language (DML) that employs
  relational operators.
• These operators are based on the concepts of
  relational algebra.

• Information is represented as two dimensional
  tables as below.

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Another Example
Flight
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Terminology
Flight
Domain set of values drawn upon by a particular
attribute Cardinality No. of Rows in a
relation Degree No. of Columns in a relation
Q Which is most likely to change?
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Terminology
Flight
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Terminology
Flight
Table type usually written as follows Flight
(Flight, Origin, Destination, Arrival)
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Data Manipulation

• Data manipulation is based on relational algebra,
  or relational calculus
• not based on traversal of hierarchies or networks
• 3 relational algebra operations
• SELECT, PROJECT, JOIN

• SELECT produces a new relation
• consisting of selected rows from an existing
  relation
• e.g. SELECT Flight WHERE Arrival gt11.00

• PROJECT produces a new relation
• consisting of a number of selected columns from
  an existing relation
• e.g. PROJECT Flight OVER Flight_, Arrival

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SELECT
11
PROJECT
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A New Relation
Tour
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Data Manipulation

• Data manipulation is based on relational algebra,
  or relational calculus
• not based on traversal of hierarchies or networks
• 3 relational algebra operations
• SELECT, PROJECT, JOIN

• JOIN produces a new relation from 2 existing
  relations joined over a common domain
• Take the first row from the first relation
  (.e.g.. Flight) and compare the attribute value
  with each attribute value in the second relation.
  If two values are identical, form a row by
  concatenating the first row with the row from the
  second relation ( strike out the repeat of the
  attribute)
• e.g. JOIN FLIGHT and TOUR OVER Flight

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JOIN
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Using a relational Database

• SQL
• can be used for stand alone queries or embedded
  in programs written in various languages
• QBE
• Powerful tool to find data
• uses the concept of examples (e.g. flight,
  destination) to find data (e.g. arrivalgt11.00)

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Advantages

• A clear concept of complex data relations
• Allow powerful data manipulation and query
  languages
• In table form - a natural representation of
  data
• Data is accessed by value and conditions so the
  database can be accessed from any position
  (relation)
• Access paths are not seen by the end user
• Representation of the data is entirely logical

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Disadvantages

• The indexes used in implementation are often
  large and require heavy storage overheads
• Operational speed of relational database systems
  can be slower than hierarchical or network
  databases
• Note do not assume that, as some have said
  (Curtis 3rd edition), this makes them unsuitable
  for high volume processing.
• (a trade-off between ideal and real-world
  conditions)