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Database Design
Sections 4 & 5Subtype, Supertype, Mutually exclusive, non-transferability, transferable, 1:1, 1:M, M:M, Redundant, Intersection entity, Barred relationship, CRUD analysis
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DJs on Demand – Partial ERD
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Reserved Words
A reserved work is one that has a specific meaning and function in the computer system, language, or database.
Examples: NULL DATE DISTINCT
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Video Store – example next slide
Identify business rules Construct ERD Class activity
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Video Store example
Read the given business scenario and walk through the steps below.
1. Examine the nouns. Are they things of significance?
2. Name each entity.
3. Is there information of interest about the entity that the business needs to hold?
4. Is each instance of the entity uniquely identifiable? Which attribute or attributes could serve as its UID?
5. Write a description of the entity.
6. Diagram each entity and a few of its attributes. Remember, entity names are always singular.
7. Identify the relationships and draw them on the ERD.
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Video ERD
featured in
•TAPE/DVDnumberformat
MOVIEid
titlecategory
ACTORstage namereal name
CUSTOMERmembership
numberlast namefirst name
phone number
a copy of
on
the feature ofrented by
the renter of
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Use Source Documents
Source document assist in identifying entities, attributes, and relationships
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Subtype
A subtype: inherits all attributes of the supertype inherits all relationships of the supertype usually has its own attributes or
relationships is drawn within the supertype never exists alone may have subtypes of its own is also known as a "subentity“
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Subtype & Supertype
Supertype (entity) can be subdivided into Subtypes
Example:
JOBmanager
clerksales
support
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Attributes
Supertype can have attributes at supertype level
Attributes at supertype level are for all subtypes
Subtypes can have own attributes for only that attribute
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Example
EXAM#number
*description*weight*date
*grade for written component
MIDTERM*grade for
oral presentation
FINAL*extra pts for opt. paper
QUIZ
BONUS QUESTIONS
STUDENT
included in
include
taken by
takes
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Global Fast Food Employees STAFF is an entity with supertypes What are the attributes of STAFF?
#id *first name *last name *DOB *salary
What subtypes exist in STAFF? ORDER TAKER *overtime COOK *training OTHER
Are there any attributes in any of the subtypes?
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Example: an incorrect subtypes in the illustration.
AUTOMOBILE subtypes are not mutually exclusive
AUTOMOBILE possible solution
BUILDING subtypes have problem of not being exhaustive, should be minimum of 2 types
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Subtype Needs Exhaustive:
Every instance of the supertype is also an instance of one of the subtypes.
Treating all parts or aspects without omissions. 2 or more items
Mutually exclusive: Every instance of the supertype is of one and
only one subtype. A relationship that presents choices which are
unable to be true at the same time.
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Correctly identify subtypes
When considering supertypes and subtypes, you can use three questions to see if the subtype is correctly identified:1. Is this subtype a kind of supertype?2. Have I covered all possible cases?
(exhaustive)3. Does the example fit into one and only
one subtype? (mutually exclusive)
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Nested subtypes
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Documenting Business Rules
Source documents assist in identiying: entities attributes relationships Some business rules are “implemented
by programming”
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Business Rules Recognize if Business rule is implemented
by the data model or by programming Example:
Only an employee with a title of manager can manage other employees
The event planner must contact the customer before the DJ can be assigned
Business rules can be changed as society changes or business leaders feel change is needed
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Business rule types Structural
Must be entered at time of initial data entry Example: When ORDER entered a staff id must
be entered Does not require code written to enforce
Procedural Work flow related Programmed validation is procedural constraint
Generally need to write code Example: Commission can not be > 25% of
salary
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Structural Business Rule examples:
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Procedural Business Rule example:
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Discussion Students must have studied algebra and
geometry in order to sign up for trigonometry. Could you represent this in the ERD?
• How would you implement this with programming?
• If the student had taken the subjects, can you think of an additional business rule that a school may want in this scenario?
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Each entity has
Name Optionality Cardinality Transferability
Example on next slide.
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Relationship Transferability
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Relationship transferability Rules established for the purpose such as
safety, efficiency, or increased profit Example: Airline tickets
can be transferable or non-transferable?
Non-transferable means also not updatable symbol used to denote non-
transferable blood type is non-transferable
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Non-transferable relationship
Orders are non-transferable Resolution would be to cancel order
and create new order
CUSTOMER ORDER
place
placed by
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Example 5.1 exercise 1 a
Each town may be the birthplace of many people. Each person must be born in one and only one town.
PERSON TOWNbirthplace for
born in
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Relationship types
One to One (1:1) One to Many (1:M) Many to Many (M:M)
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One to One relationship
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One to One relationship
Usually few of these relationships Often mandatory at one side If mandatory at both ends be aware,
it might be a error. Possible should be only one entity not two
PERSON BYCYCLE
ride
ridden by
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One to One relationship
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One to Many relationship
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One to Many
Most common type are mandatory on one side and optional on the other
Mandatory at both ends models entities that cannot exist without each other
ORDERORDER TYPE
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One to Many example
TAX RETURNTAX REPORT
LINE
BAND MUSICIANcomposed of
belong to
composed of
entered in
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Many to Many relationship
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Many to Many
Often common in initial version of ERD
Later stage of design converted to other type
STUDENT TEACHERlearn from
educate
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Redundant relationship
Redundant relationship is one that can be derived from another relationship in the model
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Many to Many Relationship Resolution
All M:M relationships must be changed to a set of 1:M and M:1 relationship
Insert a transition entity between Occurs when you go from conceptual
model to design model
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Example: M:M with Intersection Entity
STUDENT COURSE
REGISTRATION
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Resolving M:M with Intersection
Create an intersection entity If you can’t find a good intersection
entity create one by concatenate two entities together
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Example
PARTNER EVENT
WORK ASSIGNMENT (PARTNER_EVENT)
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CRUD analysis
Create ER models that reflect all business rules gathered during the interview process Identify the create, retrieve, update, and
delete (CRUD) requirements of the business
Validate your ER model by performing a CRUD analysis
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CRUD requirements CRUD analysis will help you:
Translate between the consultant and the client. Provide a guideline for the DBA who will
eventually build the database. Check that you haven’t missed any entities or
relationships in your data model. Check to make sure that you haven’t modeled
anything that the business does not require.
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What is CRUD?
CRUD is an acronym for create, retrieve, update, delete.
Check for completeness and accuracy.
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Create keywords
Look for these words when you interview the client.
INPUT ENTER LOAD IMPORT RECORD CREATE
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Retrieve keywords Look for: VIEW REPORT BRING UP PRINT FIND READ LOOK UP
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Update keywords
Look for: CHANGE MODIFY ALTER UPDATE
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Delete keywords
Look for: DISCARD REMOVE TRASH PURGE DELETE
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CRUD analysis
If you have a business function that has no entity to CRUD against, then your data model may be incomplete. Similarly, if you have entities in your ERD that are not touched by any CRUD (there is no business function that creates, retrieves, updates, or deletes from it), then you may not need that entity in your model.