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					Picking out ERD Entities

     14th March 2011
       Steps in Constructing ERDs
1.   Read and re-read the narrative
2.   Make assumptions
3.   Identify the entities
4.   Define the relationships between the entities
5.   Identify the attributes of each entity
6.   Identify the primary key of each entity
7.   Identify the required foreign keys
        Identifying the Entities
• Identifying entities in ERDs and DFDs is a
  completely different process

• ERD entities are things of interest to a system
  that data will be stored about
• DFD entities deal with external people or
  groups that interact with a particular system
  or process
• Sometimes an ERD and DFD entity will be the
  same.
  – A customer entity in a DFD may interact with an
    online booking system such as IrishRail.ie. The
    processes a customer may undergo maybe the
    following: Selecting travel time and date; Selecting the
    ticket type; Giving credit card details to process
    payment and so on!

  – An ERD for that system will include a Customer ERD
    entity. In order to book tickets online the customer
    must have an IrishRail.ie account. To create the
    account the database might store customer details
    such as username; email address; password; credit
    card details and so on in a Customer entity
• Sometimes a customer entity in a DFD will not
  mean a customer entity in an ERD.
  – A customer entity in a DFD may interact with an in-
    store shopping process such as Argos! The processes a
    customer may undergo at Argos maybe the following:
    Filling a order slip with selected product(s); Receiving
    a ticket to collect product; Payment for goods and so
    on!

  – An ERD for that system does not include a Customer
    ERD entity. In order to buy a product from Argos the
    customer does not have to register their details.
    Anybody can go into Argos and pick a product(s) from
    the catalogue and purchase it. Argos does not care
    about storing the customer details. Entities that they
    care about are likely product; category; order
   Picking entities from Narratives
• Use your personal experience to go through
  each step of the narrative. Try to think about
  what logically happens. Identify where
  information or data must be captured.

• In a supermarket, what data does a system
  manage. Typically their systems don’t store
  customer details each time they purchase
  groceries. However, what happens when a
  product barcode is scanned at checkout.
              Picking entities

The High Street is a new supermarket in Grafton
Street. They have a newly developed system
that stores all their stock item and related
details. The system caters for managing stock
levels. A customer can use the High Street
contemporary wooden enviro baskets to carry
their selected products to checkout. When a
product is added to a checkout transaction the
system adjusts the quantity of stock. In order to
keep efficient track of items the products are
stored in categories in the system.
    Picking entities – Try to remove
          unnecessary details
The High Street is a new supermarket in Grafton
Street. They have a newly developed system
that stores all their stock item and related
details. The system caters for managing stock
levels. A customer can use the High Street
contemporary wooden enviro baskets to carry
their selected products to checkout. When a
product is added to a checkout transaction the
system adjusts the quantity of stock. In order to
keep efficient track of items the products are
stored in categories in the system.
  Picking entities – Identify Potential
                 Entities
The High Street is a new supermarket in Grafton
Street. They have a newly developed system
that stores all their stock item and related
details. The system caters for managing stock
levels. A customer can use the High Street
contemporary wooden enviro baskets to carry
their selected products to checkout. When a
product is added to a checkout transaction the
system adjusts the quantity of stock. In order to
keep efficient track of items the products are
stored in categories in the system.
             Potential entities
•   Stock Item
•   Product
•   Customer
•   Checkout Transaction
•   Category
             Potential entities
• Stock Item: Same as product
• Product: System must store details about
  products they sell. How does the barcode scanner
  know what product is what!
• Customer: The customer does not have to
  register details or login before they can make
  purchases
• Checkout Transaction: To give a receipt the
  system must have some way of presenting data
  about selected products and the total paid and so
  on
• Category: As per the narrative, products are
  stored in categories for efficiency
“Nice Gifts” is a Cork City gift shop specialising in crystal and
china products. The shop sells a wide range of products,
which it periodically orders from a number of suppliers in the
Cork area. These products are offered for sale to customers
who enter the shop. A customer will enter the shop and
purchase a product. The staff will subsequently check stock
levels of the particular product and if levels are low, will
submit an order to the relevant supplier. “Nice Gifts” keeps
record of every order made to suppliers.

“Nice Gifts” also provides an additional service to customers.
When customers require a product that is currently not sold
by the shop, “Nice Gifts” will take the customers details and
order the particular product. Shop staff will then use these
details and order the particular product. Shop staff will then
use these details to contact the customer when the product
arrives on premises.
          Take the first paragraph
“Nice Gifts” is a Cork City gift shop specialising in
crystal and china products. The shop sells a wide
range of products, which it periodically orders from
a number of suppliers in the Cork area. These
products are offered for sale to customers who
enter the shop. A customer will enter the shop and
purchase a product. The staff will subsequently
check stock levels of the particular product and if
levels are low, will submit an order to the relevant
supplier. “Nice Gifts” keeps record of every order
made to suppliers.
    Potential Entities from 1st paragraph
•   Product
•   Order
•   Supplier
•   Customer

• Determine which of these are entities!
      Take the second paragraph
“Nice Gifts” also provides an additional service
to customers. When customers require a
product that is currently not sold by the shop,
“Nice Gifts” will take the customers details and
order the particular product. Shop staff will then
use these details and order the particular
product. Shop staff will then use these details to
contact the customer when the product arrives
on premises.
    Potential Entities from 2nd paragraph
•   Product
•   Staff
•   Supplier
•   Customer
•   Premises

• Determine which of these are entities!
               In-Class Exam
• 3 Questions – You must do 2
• 1st is the ERD question (MANDATORY) (70%)
• 2nd and 3rd will require short brief notes (30%)
          Question 1: ERD Tips
• Neatness will be rewarded! Make full use of the
  additional scrap paper provided.
• Give your ERD a title!
• Be consistent!
• Make sure to follow the provided instructions
• Read the narrative a few times and make sure
  you understand it.
• Try to storyboard for yourself what happens in
  the narrative scenario (e.g. Does coffee dock
  require you to login before you can buy coffee?)
• Make assumptions explicit and include them on
  the answer sheet (NOT on the scrap paper)
         Question 2 and 3: Tips
• You will have an option to choose between 2
  very short questions.
• Try to be concise – do not waffle. Use bullet
  points where possible.
• Include an illustrative example where possible

				
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posted:7/16/2011
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