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Database design conceptsinfo1408
Lecture 8
Worked example of E:R Modelling
Objectives
We will use a simple scenarioWe will identify the entitiesIdentify the relationships between the entities ( if any)For each relationship the multiplicity ( or degree) and optionality will be determined
To create an E:R Model
You would use the results of your fact finding to identify the entities ( as we did last week)In this case we will use a scenario
Identify the entities from the scenario
The Scenario
A small Cinema club runs a number of cinemas throughout the region. The cinema shows films. A film is shown several times over a week ( or more) known as a showing. Each film can have several showings. Members book tickets for a particular showingIdentify the entities
A small Cinema club runs a number of cinemas throughout the region. The cinema show films. A film is shown several times over a week ( or more) known as a showing Each film can have several showings. Members book tickets for a particular showing.
Each entity is represented as a Box
Refer back to the scenario and identify the relationships
The cinema show films.
Therefore there is a relationship between cinema and film Shown by a line- named shown at
Identify the properties of the relationship
How many cinemas can a film be shown at? How many films can a cinema show?
Does a cinema have to show a film in order to be on our database as a cinema?Does a film have to be shown at a cinema in order to be on our database?
How many cinemas can a film be shown at? Many How many films can a cinema show? ManySo the relationship is many to manyDoes a cinema have to show a film in order to be on our database as a cinema? No- optionalDoes a film have to be shown at a cinema in order to be on our database? Yes- mandatory
A film is shown several times over a week ( or more) and in several of the clubs cinemas. Each film can have several showings.
Showing and filmRelationship name-screens
A showing shows how many films?A film can have how many a showings?
A film must have a showing to be on database?A showing must have a film to show to be on the database?
A showing shows how many films? 1A film can have how many a showings?Many
A film must have a showing to be on database? Yes-mandatoryA showing must have a film to show to be on the database? Yes -Mandatory
Add to existing diagram
Members book tickets for a particular showing
How many bookings can a member make for a showing?A Showing has bookings made by how many members?
A showing must have a booking ?A member must have made a booking?
How many bookings can a member make for a showing? manyA Showing has bookings made by how many members? many
A showing must have a booking ? No- OptionalA member must have made a booking? No -optional
Add to diagram
Summary
The many to many relationships are not appropriate to the final design- we will look at what to do with them next week.We have taken entities and identified the relationships between the entities.Relationships have been described and recorded using the prescribed notation.
References
Data Analysis for database Design By D R Howe