1. Data Modelling Introduction
(special thanks to Janet Francis for this presentation)
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2. Aims Introduce Data Modelling Introduce the concept of business rules
Introduce the concept of relational databases and associated rules (Codd’s laws)
Introduce the Entity Relationship Diagram
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3. Data Modelling There are many types of data model
We will be using the Entity Relationship Model
The model provides an entity relationship diagram (ERD) which can easily be translated into a number of related tables for a relational database.
The underlying concept of storing data in a series of related tables allows for more flexible information retrieval.
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4. The Entity Relationship Diagram
Comprises
Entities
Relationships
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5. Entities Entities are represented as a box Supplier
Entities are things about which data needs to be stored.
They differ from the external entities in the process model in that they do not interact with the system - instead, the data they store is used by the system.
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6. Attributes An attribute describes an entity
Eg. Attributes for the entity “Product” might be:
Type
Colour
Size
Shape
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7. Attributes, Entities and the Database
The Entity name will become the table name
The attributes will be the column names
Attributes will have a type associated with them
eg. Text, Character, Integer, Date
This type determines how they are stored and the
operations which can be performed
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8. Relationships
An entity cannot exist in the system without being related to another entity.
The relationship between two entities is determined by a set of business rules and/or assumptions
Relationships are drawn on the ERD as a line.
Relationships can be described in three ways:
Multiplicity (also known as Cardinality, Degree, Order)
Optionality (Whether participation in the relationship is optional or not)
Name
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9. Multiplicity (Cardinality)
The relationship can be defined as
1:many
1:1
Many to Many
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10. Optionality The relationship can be defined as 0:many 0:1 0 to Many
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11. Name
A relationship must be named. The relationship name will contain a verb.
Naming is important because between two entities, there might be more than one relationship.
Eg The Entities Staff and Module
One member of staff leads the module
Members of staff teach the module
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12. An example Business Rules
A supplier supplies at least one and possibly many products
Each product has only one supplier
Supplier
Product
supplies
Is supplied
Multiplicity 1:many
No Optionality
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13. Another example Business Rule
A person may give one or more contact telephone numbers
supplies
Is supplied
Person
Contact telephone No.
Multiplicity 1:many
Optionality on one side – the number (if given)
will belong to someone
but not everyone will choose to give a contact number.
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14. Laws of the relational model
Edgar Codd devised the Relational Model based on mathematical concepts.
Mathematical concepts are defined by rules (laws).
The Relational Model has 12 rules
However, not all the rules are in common use in popular RDBMS’s
We will pay particular attention to rule 2 and rule 10.
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15. Rule 2: Aim - Entity Integrity
Ensuring entity integrity is the aim of one of the basic rules which is applied to relational databases
Ensuring entity integrity means ensuring that there are no duplicate records within the table
To make sure that Entity Integrity is preserved, each entity must have a special attribute which uniquely identifies it.
the field that identifies each record within the table is known as a primary key – the primary key MUST
Be unique
Have a value (I.e. not be null)
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16. Rule 10: Aim - Referential Integrity
Ensuring referential integrity means ensuring that
data is consistent between related tables
enforced by the existence of a foreign key (which is related to the primary key in the related table)
The foreign key MUST contain a value which is present in the primary table.
Integrity constraints are used to enforce this.
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