DATABASE DESIGN
ERD
• Entity Relational Diagram (ERD)
• Visual display of tables and their relationships for
a database
• Used for
“Relational
Modeling”
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ERD cont.
• SQUARES indicate
TABLES
• LINES between them
indicate the different relationships
• “FEET” at end of line,
indicates a “MANY” relationship
• -1 CUSTOMER has MANY “receipts”
• -1 STAFF MEMBER sells
MANY “receipts”
• -1 PRODUCT can go into many different “receipts”
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• Each product has to be INDIVIDUALLY listed
ERD cont.
• IF I wanted to know who sold me the order, I have to go to the receipt
(order ) , in the “RECEIPT” table, I’ll find who the STAFF MEMBER is who
sold me that receipt
•
IF I wanted to know what product a CUSTOMER has ordered, I have to
go to the receipt (order ), in the “PRODUCT” table
• -NO DIRECT relationship BETWEEN :
• CUSTOMER & STAFF, or
• CUSTOMER & PRODUCT
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What are Tables or Entities
• Used to represent things in the “real world” that is of
significant interest to the user
• i.e. “EMPLOYEES” might be an
“Entity” in a company database
Employees
• NOUNS in a “Business Narrative”
• each table is called an “entity” and the columns are
referred to as an “attribute”.
• within a table , each row is called a “record” and it
represents 1 particular instance of its “entity” (not
unlike the flat file).
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Entities Continued
• Another example:
• A database regarding music
• Artist & Song are both tables
• “Performs” is the relationship (VERB) (usually is a “supporting
document” which explains the relationship)
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Entities and Their Attributes
• “Entities” represent something of importance
• “Entities” are made up of several “Attributes” (the
columns), which describe the tables:
ARTISTS
SONGS
ArtistName
SongTitle
Gender
Length
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Entities Summarized
• Each ENTITY is made up of rows and columns
• Columns are called fields or attributes of a database
• Rows are called records or an instance of an entity
• If you have repetitive VALUES to 1 attribute, —create a:
“JOINER TABLE” or “LOOKUP TABLE”
• make this attribute a “Foreign” key
• Create another table with same attribute
• i.e. a: “Shoe Table” with “colours”
• “Colours” will be a “Foreign” key, here: with “ColourID”
• primary key in a new table called: colours, with “ColourID”
• 8-10 attributes per TABLE, after that create another table
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Entities Summarized cont.
• OR: When a table has the SAME VALUE
REPETITIVELY for that attribute, you might need a
“LOOKUP TABLE”:
1. People might misspell, the value, then you can’t do a:
SEARCH
 i.e. LEVEL OF EDUCATION
 “Masters”
 What if 1 person spells it: “Masters”
o Another person “Master”
o Another person Masters of Education
2. You will have multiple occurences of 1 VALUE
which takes up too much space in your database
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Steps in Database Design
1.
Complete a “Needs Assessment” (note: this is a huge process
that is not covered in this course)
• Basically, you speak with end users, managers, existing
documents and think “outside the box”
• If, in the design of the database, there is multiple occurrences of
data (think of our “gender” example) this is an indicator that
another table should be created.
2.
Create a “Business Narrative”
3.
Identify entities
4.
Identify attributes & keys
5.
Identify relationships
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Business Narrative
• Vital link between database designer and the end users
• Purpose is to make clear the “data requirements”
• The story, business rules, purposes of
database; what kind of info it will hold
• Will have to go over the DESIGN multiple times/days to make
sure the DESIGN is correct
• Get other people/users to comment on design
• Example:
• “Algonquin college employs many instructors to deliver various
programs comprised of many different courses. “
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Starting an ERD
• Identify the “entities”
• Look at NOUNS of the “Business Narrative”
“Algonquin college employs many instructors to deliver various
programs comprised of many different courses.”
programs
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instructors
courses
When the list of Entities Seems Complete
• Ask about each “entity”:
1. Is it significant?
• List only entities that are important to your
database users and that are worth the trouble and
expense of computer tabulation.
2. Is it generic?
• List only types of things, not individual instances
• “EMPLOYEES”, “INSTRUCTORS” are generic of many
different instances—we don’t say “Joe”
3. Is it fundamental?
• List only entities that exist independently and do not
need something else to explain them.
• “Courses”-explains itself, you don’t need further
elaboration
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RELATIONSHIPS
Three Basic Types of Relationships
• One to One (1:1)
• One to Many (1:M)
• Many to Many (M:M)
• The 1 is the “PARENT”, and the M is the “CHILD”
• Called the CARDINALITY of the relationship
• The cardinality indicates the MAXIMUM number of RELATIONSHIPS
between the entities,
• Whenever you see: “MANY”—this will be the FOREIGN KEY
One to One Relationships
• 1:1 relationships exists when a single record in one table has
1 and only 1 corresponding record in another table, and vice
versa
• VERY RARE
• When you created a table for something, when it really
should only have been a “field”
• The 1 should really have been an “attribute” of the other
(table/entity) an attribute IMPROPERLY DEFINED as an
entity
• i.e. 1 customer, 1 address
• “Address” should have been the attribute of the “Customer” table
One to One Example
• A voter can cast only one vote in an election.
• A ballot paper can belong to only one voter.
• So there will be a 1:1 relationship between a Voter and a
Ballot Paper.
Voter
Ballot
One to Many Relationships
• MOST COMMON
• 1:M or M:1
• Relationship exists when a record can relate to 1 OR MORE
records in a 2nd table but..
• A record in the 2nd table can ONLY relate to 1 record in the 1st
table
One to Many Example
• A person can own more than one car. A car can only have one
owner.
Owner
Vehicle
Many to Many Relationships
• M:M
• Relationship exists when 1 record in either table can relate to
more than 1 record in the other table
• These should be eliminated
• 2 related “PARENT” tables, but their relationship is NOT “direct”
• Instead, it needs to be related through an additional “CHILD” table
Many to Many Example
• A student can have more than one professor; the same
professor can have many students
Student
• http://www.youtube.com/watch?v=0lSLI7q-hVE
Professor
Resolving Many to Many
• M:M is not allowed, as an attribute could have more than 1
value:
• A “Multi-Valued Attribute”
• Difficult to generate reports, or do searches
• Would have to repeat attributes multiple times
• Instead, we create a “JUNCTION” or “JOINER ENTITY”
• The name of the joiner entity is often a COMBINATION of the
2 entities
Student
Stud_Prof
Professor
Many to Many Relationships
Resolving Many to Many Relationships By
Creating A Junction Or Joiner Entity
• http://www.youtube.com/watch?v=NvrpuBAMddw
SHEETS OF ICE
• 1 arena with many sheets
• ARENA ID-primary key in ARENA table
• Add a FOREIGN KEY to SHEETS OF ICE (AREN_AID)
• Can a sheets of ice belong to many arenas? NO—so we
know it’s a one-to-many
• TEAMS table-PRIMARY KEY: TEAM ID to PLAYERS table
FOREIGN KEY: TEAM ID
Relationship Review
Normalization
•
Refining the ERD
•
Task of analyzing entities and the relationships created have been
formalized into a process called normalization.
•
Should resolve all your relationships to One to Many (1:M)
relationships
• Eliminate all One to One’s (1:1)
(M:M)
and Many to Many’s
Sample Business Narrative
• CD Collection example
• I have many cd’s by various artists. I like all types of music
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Sample Business Narrative
• Courses example:
• A course can have many different learning objectives and each
learning objective has various examples and a method of testing.
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Sample Business Narrative
• New Home Builders example:
• Bamarack is a builder of new homes in the Ottawa area. They
subcontract all the work that needs to be completed for the build
of a new home. Each subcontractor has a specialized trade.
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We Build a Business Narrative
• Create a business narrative for your shoe collection at home
STYLE
STYLE ID
STYLE NAME
1
SPORTY
2
SPORTY
3
DRESSY
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Attributes for Our Shoe Scenario
• Identify the attributes for all entities identified for our shoe
inventory database
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Selecting Attributes
• Attributes are the fields/columns that describe an entity
• Attributes can be:
• A Characteristic
• A Quality
• A Feature
• A Fact
• An attribute is a “non-decomposable” piece of information about an entity
• Can’t be broken down any further, only 1 single value per column
• i.e. NAME—last name, 1st name
• Attribute “Naming Conventions”
• Single word (or more than 1 word with an underscore (no spaces)
• Unique
• Can’t be a reserved word”
• i.e. “Number”, “Date” ( these are date types), OR name of column
can’t be same as name of table
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Attributes
• Identify the entity and attributes for the following business
narrative:
• ABC Property Management Inc has a building with 20 units.
Some are rented, some are vacant. Some are 1 bedroom, some
are 2 bedrooms. The rent varies. ABC Property Management Inc
keeps track of the tenant and lease information
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• ONLY WATCH UNTIL 5 minutes
31 seconds (until it mentions:
“QUERYSMITH”)
• http://www.youtube.com/watch?v=
Y-bvjtYgRVU
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REMOVE SORT
• HOME tab, in the SORT & FILTER
group, click the REMOVE SORT
button:
FILTER BY SELECTION in a “TABLE”
• HOME > SORT & FILTER group > click on the FILTER
icon >
• To FILTER:
• Click on the drop-down arrow on the COLUMN
HEADING (field) you want to filter
• Select or de-select the VALUES
you want to filter
NUMBER FILTERS
• Click on the drop-down arrow on
the COLUMN HEADING (field)
you want to filter
• NUMBER FILTERS > select one of this options:
• When you select “Less Than” or
• “Greater Than”, by default it will
• Also contain “Equal To”, so key in
• The next number UP (for GREATER than), or The next number
DOWN (for LESS than)
• I.E. If you want >200, you would have to key in >201 (as it
contains “equal to”, and will flag “200” also)
FILTER BY FORM
• SORT & FILTER group
• ADVANCED button
• FILTER BY FORM
FILTER BY SELECTION in a “QUERY”
• To filter all the rows in a table that contain a
value that matches a selected value in a row
• Right-click the VALUE you want to filter. > select:
“EQUALS” the specific value you want
• IF A FILTER IS ALREADY APPLIED:
• HOME tab, > SORT & FILTER group,
> Advanced, > click: CLEAR ALL FILTERS.
ADDING A “TOTAL” ROW IN A QUERY
• QUERY TOOLS DESIGN tab, > SHOW/HIDE
group, click the TOTALS button
• In the TOTAL row below the AMOUNT field,
click the drop-down arrow and select: MIN.
• DESIGN tab, > RESULTS group, > RUN button.
• VIEW the query in DESIGN view
HYBRID: MYITLAB
• MYITLAB HOMEWORK:
1. myitlab ASSIGNMENT:
“EXPLORING series”
Access Chapter 1- GRADER
PROJECT [HOMEWORK]
2. myitlab TRAINING:
Access VOLUME 1- “Skill-Based
TRAINING”