LECTURE 11 – DATA MODELLING
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Data Dictionaries
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Entity Relationship Diagram for Data Modelling
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Steps to Construct Entity Relationship Diagrams
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Validation of Entity Relationships Diagram
Software Project Management
Maria Petridou
DATA DICTIONARIES
It is a main method for analyzing the data flows and data
stores of data-oriented computer systems.
 The data dictionary is a reference work of data about data
(metadata)
 It collects, coordinates, and confirms what a specific data
term means to different people in the organization.
 Data dictionaries are created by making a detailed analysis of
the data flows, data stores, and processes in data flow
diagrams.
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DATA DICTIONARIES
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The data dictionary may be used for the following reasons:
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Provide documentation
Eliminate redundancy
Validate the data flow diagram for completeness and accuracy
Provide a starting point for developing screens and reports
Determine the contents of data that is stored
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DATA DICTIONARIES
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Data dictionaries may be created after the DFDs or at the same
time than the DFDs using a top-down approach.
Creating a data dictionary is an iterative and evolutionary process
that should be carried out in parallel to the systems analysis and
design.
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DATA DICTIONARIES
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Data Repository is a large collection of project information
including:
 Information about system data
 Procedural logic
 Screen and report design
 Relationships between entries
 Project requirements and deliverables
 Project management information
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DATA DICTIONARIES
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Data dictionaries contain:
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Data flow
Each data flow should be defined with descriptive information and it's
compound with structure or elements.
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Data structures
They are a group of smaller structures and elements. Specific arrangements
of data attributed (elements) that define the organization of a single
instance of a data flow.
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Data Elements
The descriptive property or characteristic of an entity. In database terms,
this is a "attribute" or a "field."
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Data stores
A inventory of data. The whole of the data in a small system. A database!
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ENTITY RELATIONSHIP DIAGRAMS FOR DATA
MODELLING
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Data modelling focuses on data flows and data stores represented in the
data flow diagrams.
Data models can be of two types:
Logical data models – describe logical organisation and presentation of data.
A technique for logical data modelling is the entity-relationship diagrams.
 Physical data models – produced in the design phase, give implementation
details about how data will actually be stored in databases and files.
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An Entity-Relationship Diagram (ERD) shows how the data that flows in
the system is organised and used.
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ENTITY RELATIONSHIP DIAGRAMS FOR DATA
MODELLING
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The elements of an ERD are the following:
 Entity – multiple instances about which the data is collected.
 Attributes – specific details or information about an entity.
 Relationships – associations between entities. A relationship has:
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Cardinality - The number of instances of entity B that can be associated
with each instance of entity A.
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Minimum Cardinality or Modality - The minimum number of instances
of entity B that may be associated with each instance of entity A.
 Maximum Cardinality - The maximum number of instances of entity B
that may be associated with each instance of entity A
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ENTITY RELATIONSHIP DIAGRAMS FOR DATA
MODELLING
Relationships
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A one-to-one relationship (1:1) exists when the maximum cardinality for
each entity in the relationship is 1.
A one-to-many (1:N) relationship exists when the maximum cardinality on
one side is 1 and the maximum on the other side is many.
A many-to-many (M:N) relationship exists when the maximum on both
sides is many.
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ENTITY RELATIONSHIP DIAGRAMS FOR DATA
MODELLING
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Types of entity relationships and example of their representation:
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ENTITY RELATIONSHIP DIAGRAMS FOR DATA
MODELLING
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Implement One-to-One Relationships
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Implement One-to-Many Relationships
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In a relational database, one-to-one relationships between entities can be
implemented by merging the two tables/entities together.
Place the primary key of the entity that can occur only once as a foreign key
in the entity that can occur many times.
Implement Many-to-Many Relationships
In a relational database, many-to-many relationships between entities can
be implemented by creating a new entity that links the other two entities
together. In most cases, the primary key of the new entity consists of the
primary keys of the entities that it is relating together.
 Spit it into two one-to-many relationships!
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ENTITY RELATIONSHIP DIAGRAMS FOR DATA
MODELLING
Student
Module
Student ID (pk)
Student first name
Student last name
…
Student
Student ID (pk)
Student first name
Student last name
…
Module ID(pk)
Module name
Module Credits
…
Module
Enrollment
Module ID (pk) + (fk)
Student ID (pk) + (fk)
in
Module ID(pk)
Module name
Module Credits
…
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ENTITY RELATIONSHIP DIAGRAMS FOR DATA
MODELLING
Example of an ERD for the doctor’s office appointments system (see DFD in
handouts of Lecture 9).
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STEPS TO CONSTRUCT ENTITY RELATIONSHIP
DIAGRAMS
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Like constructing DFDs and data dictionaries, the construction of ERDs is
an iterative and evolutionary process that requires refinement as a
better understanding of the system to be developed is achieved.
1.
2.
3.
4.
5.
Identify entities – use DFDs and use cases to identify multiple instance
entities.
Add attributes – requires detailed analysis and perhaps use of
requirements gathering techniques.
Assign identifiers or Primary keys – Each entity type must have an
attribute or set of attributes that distinguishes one instance from other
instances of the same type.
Foreign Keys - The attributes of a foreign key in one entity are the
attributes of a primary key in another entity.
Identify Relationships – involves establishing the cardinality and the
modality of each relationship.
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STEPS TO CONSTRUCT ENTITY RELATIONSHIP
DIAGRAMS
Example: Create a logical data model (based on an ERD) for the
University Registration System (see handouts for Lecture 9).
1. Identify Entities
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STEPS TO CONSTRUCT ENTITY RELATIONSHIP
DIAGRAMS
2. Add Attributes
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STEPS TO CONSTRUCT ENTITY RELATIONSHIP
DIAGRAMS
3. Assign Identifiers
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STEPS TO CONSTRUCT ENTITY RELATIONSHIP
DIAGRAMS
4. Identify Relationships
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STEPS TO CONSTRUCT ENTITY RELATIONSHIP
DIAGRAMS
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Important guidelines when creating ERDs:
 Use consistent labels and names
 Avoid unnecessary attributes
 Break-down the model if needed
 Determine cardinality and modality correctly
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VALIDATION OF ENTITY RELATIONSHIP
DIAGRAMS
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Two available techniques to validate ERDs are:
Normalisation and Balancing DFD/ERD
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Normalisation – set of rules applied to the ERD in order to detect
problems in the data model. The data model is checked against 1st
Normal Form, 2nd Normal Form and 3rd Normal Form.
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Balancing DFD/ERD – there should be a balance or correspondence
between the data elements in the DFD and the ERD (data stores-entities
and data flows-attributes). This may provoke changes to the DFD and the
ERD!
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VALIDATION OF ENTITY RELATIONSHIP
DIAGRAMS
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The DFD and ERD are balanced when:
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There is a one-to-one correspondence between data stores in the
DFD and entities in the ERD.
Data flows in the DFD are made of data elements corresponding to
attributes in the ERD.
Reading: (Kendall&Kendall,chapter 9) and (Dennis &Wixom,chapter 7).
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