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ERD Solution and Supplementary Notes. Note. The following is not the only solution for the in-class ERD. Many different solutions were perfectly valid due to the underlying assumptions made about the narrative. Make your assumptions explicit!!
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Note • The following is not the only solution for the in-class ERD. Many different solutions were perfectly valid due to the underlying assumptions made about the narrative. • Make your assumptions explicit!! • Also the in-class exam theory based questions for both the in-class exam and the in-class exam resit are included at the end of this presentation.
EntNightsOut ERD Narrative • EntNightsOut is a Cork based company recently set up to promote up and coming entertainment artists. They run a number of events on a monthly basis throughout the entire year and during the summer months they may even run 3-4 events per week. Each EntNightsOut event has at least 3 entertainment artists performing. There are many types of entertainment performers from musicians to comedians to jugglers and so on. • Each event is held in one of the many venues in Cork City that are renowned for their culture and lively atmosphere. Indeed, EntNightsOut have received much praise from local and national press for their policy of ensuring to send “business” (i.e. by way of an event) to many different locations. • Furthermore, a host is always present at each event to introduce performers and also to ensure event sponsors are given full recognition. Most events depending on the time of year and the line up of entertainment artists are sponsored by at least one company. • Tickets can be purchased in the EntsNightOut store in Patrick Street, Cork City. Customers receive a stamp on their loyalty card (similar to that of a coffee shop) after each purchase. When a customer collects 7 stamps they receive a 50% discount on the next event purchase at the EntsNightOut till.
EntNightsOut ERD Solution • A venue may hold one or many events throughout the year • An event will be held in one and only one venue. • Note: A valid assumption made by a few people was that an entnightsout event made occur in many different venues during the one night. As such, that assumption would then create a many-to-many relationship!
EntNightsOut ERD Solution • Each event will have one host presenting the event. Some made valid assumption that more than one host may be present at larger events and as such created another many-to-many • A host may be present at one or many events throughout the year!
EntNightsOut ERD Solution • There are many tickets for each event • A ticket is for one event. Note: An assumption could be made that tickets may be for more than one event (i.e. a ticket may be valid for an event tonight and the following event tomorrow night). E.G. Oxegen Tickets
EntNightsOut ERD Solution • An event may have one or more artists/performers performing. • An artist/performer may perform at one or more events during the year
EntNightsOut ERD Solution • An event is sponsored by at least one sponsor. It may be sponsored by many sponsors. • A sponsor may sponsor one event or many events during the year.
Customer Not an Entity • Customer is not an entity in this ERD! • Customers walk into a store and purchase tickets. They do not have to give their personal information to buy a ticket. • Although they collect stamps on a loyalty card, the narrative clearly states it is a loyalty card system similar to that of a coffee shop. Example: An individual does not give personal details in order to collect stamps on their coffee dock loyalty card!
Customer Not an Entity • Having read a narrative and picked out potential entities you should ask the following question of each entity: • Does data or information need to be stored about this entity for the system to function.
Associative Entities & Compound Keys • Whenever you identify a many-to-many relationship you must decompose it and create what is called an associative entity • The unique identifiers or keys in the associative entity are known as a compound key. The compound key for the many to many relationship is the primary key of each entity making up the many-to-many relationship!
Associative Entities & Compound Keys • Example: Many-to-Many relationship between Event and Artist. One or Many Artists play at each Event. An Artist may play at One or Many events throughout the year. • The compound key for the associative entity is made up of the foreign key of the Artist and of the Event entity (i.e. Event_ID and Artist_ID)
Identifying Many-to-Many • Even if you cannot remember what to do to remedy a many-to-many relationship you should at least make it known that you understand that it is a problem. • Circle the many-to-many relationship and write a note indicating that it is a problem. • Many people lost marks by not doing this in the in-class exam
Exam • Question 1 is mandatory and will require you to read a narrative and subsequently construct an ERD. • Read the narrative carefully and question whether or not your potential entities require data to be stored about them or not! • The second part of question 1 is theory based. Review the questions you saw in the in-class exam and these notes.
Theory Questions from the ERD in-class exam! Q2: With regards to ERDs, write brief notes to explain what is meant by (i) Cardinality; and (ii) Associative Entities. Support your answer with an example and diagram where possible. Q3: Briefly outline the rationale for data modelling such as constructing ERDs.
Theory Questions from the ERD in-class exam RESIT! Q2: With regards to ERDs, write brief notes on two of the following ERD related elements: (i) Cardinality (ii) Associative Entities (iii) Compound Key Support your answer with an example and diagram where possible. Q3: Briefly outline the interdependence between the systems analysis phase and the systems design phase of any information system development initiative.
