ITEC 3220M
Using and Designing Database Systems
Instructor: Prof. Z. Yang
Course Website: http://people.yorku.ca/~zyang/itec
3220m.htm
Office: Tel 3049

Supertypes and Subtypes
• Generalization hierarchy: depicts relationships between higher-level supertype and lower-level subtype entities
• Supertype: contains the shared attributes
• Subtype: contains the unique attributes
• Inheritance:
– Subtype entities inherit values of all attributes of the supertype
– An instance of a subtype is also an instance of the supertype
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Supertypes and Subtypes (Cont’d)
Supertype/ subtype relationships
Attributes shared by all entities
General entity type
SUPERTYPE
SUBTYPE1
Attributes unique to subtype1
SUBTYPE2
Attributes unique to subtype2
And so forth
Specialized version of supertype
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Supertypes and Subtypes (Cont’d)
• Disjoint relationships
– Unique subtypes
– Non-overlapping
– Indicated with a ‘G’
• Overlapping subtypes
– An instance of the supertype could be more than one of the subtypes
– Indicated with a ‘Gs’
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Generalization Hierarchy with
Overlapping Subtypes
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Logical Database Design and
Normalization of Database Tables

In this chapter, you will learn:
• How to transform ERD into relations
• What normalization is and what role it plays in database design
• About the normal forms 1NF, 2NF, 3NF, BCNF, and 4NF
• How normal forms can be transformed from lower normal forms to higher normal forms
• Normalization and E-R modeling are used concurrently to produce a good database design
• Some situations require denormalization to generate information efficiently
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• Step one: Map regular entities
– Each regular entity type in an ER diagram is transformed into a relation
– The name given to the relation is generally the same as the entity type
– Each simple attribute of the entity type becomes an attribute of the relation and the identifier of entity becomes the primary key of the corresponding relation
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Example
STUDENT
Student_ID Student_Name Other_Attributes
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Transforming ERD into Relations
(Cont’d)
• Step two: Map weak entities
– Create a new relation and include all of the simple attributes as the attributes of this relation. Then include the primary key of the identifying relation as a foreign key attribute in this new relation. The primary key of the new relation is the combination of this primary key of the identifying relation and the partial identifier of the weak entity type.
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Example
Employee_ID Employee_Name
Dependent_Name
Date_of_birth
EMPLOYEE
EMPLOYEE
Employee_ID
Has
Employee_Name
DEPENDENT
Dependent_Name Employee_ID
DEPENDENT
Date_of_birth
Gender
Gender
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Transforming ERD into Relations
(Cont’d)
• Step three: Map binary relationship
– Map Binary one-to-many relations
•First create a relation for each of the two entity types participating in the relationship, using the procedure described in step one.
•Next, include the primary key attribute of the entity on the one-side of the relationship as a foreign key in the relation that is on the many-side of the relationship.
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Example
Customer_ID Customer_Name
Order_ID Order_Date
1
M
Customer Submits Order
(0,N)
Customer
Customer_ID Customer_Name
(1,1)
Order
Order_ID Order_Date Customer_ID
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Transforming ERD into Relations
(Cont’d)
• Step three: Map binary relationship
(Cont’d)
– Map binary one-to-one relationships
•First, two relationships are created one for each of the participating entity types.
•Second, the primary key of one of the relations is included as a foreign key in the other relation.
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Example
Nurse_ID Nurse_Name
Centre_Name Location
Nurse
1
(0,1)
In_charge
1
(1,1)
Care
Centre
Nurse Nurse_ID
Care
Centre
Centre_Name
Nurse_Name
Location Nurse_in_c harge
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Transforming ERD into Relations
(Cont’d)
• Step Four: Map composite Entities
– First step
• Create three relations: one for each of the two participating entities, and the third for the composite entity. We refer to the relation formed from the composite entity as the composite relation
– Second step
• Identifier not assigned: The default primary key for the composite relation consists of the two primary key attributes from the other two relations.
• Identifier assigned: The primary key for the composite relation is the identifier. The primary keys for the two participating entity types are included as foreign keys in the composite relation.
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Example
Order_ID
Product_ID
Description
Order
1
M
(1,N)
(1,1)
Order
Line
M
1
Product
(1,1)
(0,N)
Order_Date
Quantity
Price
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Example
Order
Order_ID Order_Date
Order Line
Product_ID Order_ID Quantity
Product
Product_ID Description Standard_Price
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Example
Customer_ID
Name
Date
Customer
1 M
Shipment
Vendor_ID
M 1
Address
Vendor
Customer
Shipment_No
Customer_ID
Amount
Customer_Name
Shipment Shipment_N o
Vendor_I
D
Customer_I
D
Date Amount
Vendor Vendor_ID Address 19

Transforming ERD into Relations
(Cont’d)
• Step Five: Map unary relationship
– Map unary one-to-may relationship
•The entity type in the unary relationship is mapped to a relation using the procedure described in Step one. Then a foreign key attribute is added within the same relation that references the primary key values. A recursive foreign key is a foreign key in a relation that references the primary key values of that same relation.
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Employee_ID
Example
Name
Birthdate
Employee
M (1,1)
1 (0,N)
Manages
Employee
Employee_ID Name Birthdate Manager_ID
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Transforming ERD into Relations
(Cont’d)
• Step six: Map ternary relationship
– Convert a ternary relationship to a composite entity
– To map a composite entity that links three regular entities, we create a new composite relation. The default primary key of their relation consists of the three primary key attributes for the participating entities. Any attributes of the composite entity become attributes of the new relation
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Example
Patient_ID
Patient_Name Physician_ID
Physician_Name
Patient
1
Results
M
(0,N)
(1,1)
(0,N)
(1,1)
M
Patient
Treatment
Physician
1
Date
Time
M (1,1)
Treatment_
Code
1 (0,N)
Treatment
Description
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Example
Patient
Patient_ID Patient_Name
Physician
Physician_ID Physician_Name
Patient Treatment
Patient_
ID
Treatment
Physician_I
D
Treatment_C ode
Date Time Result
Treatment_Code Description
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Transforming ERD into Relations
(Cont’d)
• Step seven: Map supertype/subtype relationships
1. Create a separate relation for the supertype and for each of its subtype
2. Assign to the relation created for the supertype the attributes that are common to all members of the supertype, including the primary key
3. Assign to the relation for each subtype the primary key of the supertype, and only those attributes that are unique to that subtype
4. Assign one attribute of the supertype to function as the subtype discriminator
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Example
Employee_Name
Address
Employee_Type
Employee_Number
Hourly
Employee
Employee
Gs
Salaried
Employee
Date_Hired
Hourly_Rate Annual_Salary Stock_Option
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Example
Employee
Employee_Nu mber
Employee_N ame
Hourly_Employee
H_Employee_Number
Address
Hourly_Rate
Employee_Ty pe
Date_Hi red
Salaried_Employee
S_Employee_Number Annual_Salary Stock_Option
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Database Tables and
Normalization
• Table is the basic building block in database design
• Normalization is the process for assigning attributes to entities
– Reduces data redundancies
– Helps eliminate data anomalies
– Produces controlled redundancies to link tables
• Normalization stages
– 1NF - First normal form
– 2NF - Second normal form
– 3NF - Third normal form
– 4NF - Fourth normal form
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Need for Normalization
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• PRO_NUM intended to be primary key
• Table displays data anomalies
– Update
• Modifying JOB_CLASS
– Insertion
• New employee must be assigned project
– Deletion
• If employee deleted, other vital data lost
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Conversion to First Normal Form
• Repeating group
– Derives its name from the fact that a group of multiple entries of same type can exist for any single key attribute occurrence
• Relational table must not contain repeating groups
• Normalizing table structure will reduce data redundancies
• Normalization is three-step procedure
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Conversion to First Normal Form
(continued)
• Step 1: Eliminate the Repeating Groups
– Present data in tabular format, where each cell has single value and there are no repeating groups
– Eliminate repeating groups, eliminate nulls by making sure that each repeating group attribute contains an appropriate data value
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Conversion to First Normal Form
(continued)
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Conversion to First Normal Form
(continued)
• Step 2: Identify the Primary Key
– Primary key must uniquely identify attribute value
– New key must be composed
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Conversion to First Normal Form
(continued)
• Step 3: Identify all dependencies
– Dependencies can be depicted with help of a diagram
– Dependency diagram:
•Depicts all dependencies found within given table structure
•Helpful in getting bird’s-eye view of all relationships among table’s attributes
•Makes it less likely that will overlook an important dependency
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Conversion to First Normal Form
(continued)
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Conversion to First Normal Form
(continued)
• First normal form describes tabular format in which:
– All key attributes are defined
– There are no repeating groups in the table
– All attributes are dependent on primary key
• All relational tables satisfy 1NF requirements
• Some tables contain partial dependencies
– Dependencies based on only part of the primary key
– Sometimes used for performance reasons, but should be used with caution
– Still subject to data redundancies
37

Conversion to Second Normal
Form
• Relational database design can be improved by converting the database into second normal form (2NF)
• Two steps
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Conversion to Second Normal
Form (continued)
• Step 1: Write Each Key Component on a Separate Line
– Write each key component on separate line, then write original (composite) key on last line
– Each component will become key in new table
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Conversion to Second Normal
Form (continued)
• Step 2: Assign Corresponding
Dependent Attributes
– Determine those attributes that are dependent on other attributes
– At this point, most anomalies have been eliminated
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Conversion to Second Normal Form
(continued)
41

Conversion to Second Normal
Form (continued)
• Table is in second normal form (2NF) when:
– It is in 1NF and
– It includes no partial dependencies:
•No attribute is dependent on only portion of primary key
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Conversion to Third Normal Form
• Data anomalies created are easily eliminated by completing three steps
• Step 1: Identify Each New Determinant
– For every transitive dependency, write its determinant as PK for new table
•Determinant
– Any attribute whose value determines other values within a row
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Conversion to Third Normal Form
(continued)
• Step 2: Identify the Dependent
Attributes
– Identify attributes dependent on each determinant identified in Step 1 and identify dependency
– Name table to reflect its contents and function
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Conversion to Third Normal Form
(continued)
• Step 3: Remove the Dependent Attributes from
Transitive Dependencies
– Eliminate all dependent attributes in transitive relationship(s) from each of the tables that have such a transitive relationship
– Draw new dependency diagram to show all tables defined in Steps 1–3
– Check new tables as well as tables modified in
Step 3 to make sure that each table has determinant and that no table contains inappropriate dependencies
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Conversion to Third Normal Form
(continued)
46

Conversion to Third Normal Form
(continued)
• A table is in third normal form (3NF) when both of the following are true:
– It is in 2NF
– It contains no transitive dependencies
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The Boyce-Codd Normal Form
(BCNF)
• Every determinant in table is a candidate key
– Has same characteristics as primary key, but for some reason, not chosen to be primary key
• When table contains only one candidate key, the
3NF and the BCNF are equivalent
• BCNF can be violated only when table contains more than one candidate key
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The Boyce-Codd Normal Form
(BCNF) (continued)
• Most designers consider the BCNF as special case of 3NF
• Table is in 3NF when it is in 2NF and there are no transitive dependencies
• Table can be in 3NF and fails to meet
BCNF
– No partial dependencies, nor does it contain transitive dependencies
– A nonkey attribute is the determinant of a key attribute
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The Boyce-Codd Normal Form
(BCNF) (continued)
50

The Boyce-Codd Normal Form
(BCNF) (continued)
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An Example
GRADE( Student_ID, Student_Name, Address, Major,
Course_ID, Course_Title, Instructor_Name,
Instructor_Office, Grade)
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Normalization and Database
Design
• Normalization should be part of the design process
• E-R Diagram provides macro view
• Normalization provides micro view of entities
– Focuses on characteristics of specific entities
– May yield additional entities
• Difficult to separate normalization from E-R diagramming
• Business rules must be determined
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Higher-Level Normal Forms
• Fourth Normal Form (4NF)
– Table is in 3NF
– Has no multiple sets of multivalued dependencies
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Conversion to 4NF
Stud-ID
1126
1126
1126
1126
1126
Stud-ID
1126
1126
1126
Stud-ID Course
1126 1212F
1126
1126
1620F
1320F
Course
1212F
1620F
1320F
Course
1212F
1620F
1320F
Service
Red Cross
United Way
Service
Red Cross
United Way
Service
Red Cross
United Way
Multivalued Dependencies
Stud-ID
1126
1126
1126
Stud-ID
1126
1126
Course
1212F
1620F
1320F
Service
Red Cross
United Way
Set of Tables in 4NF
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Denormalization
• Normalization is one of many database design goals
• Normalized table requires
– Additional processing
– Loss of system speed
• Normalization purity is difficult to sustain due to conflict in:
– Design efficiency
– Information requirements
– Processing
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Exercise
Part Supplier Data
Part_
No
Description Vendor_Nam e
1234 Logic Chips Fast Chips
Smart Chips
Address Unit_C ost
Cupertino
Phoenix
10.00
8.00
5678 Memory Chips Fast Chips
Quality Chips
Smart Chips
Cupertino
Austin
Phoenix
3.00
2.00
5.00
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Exercise(Cont’d)
• Convert the table to a relation in first normal form
(Named Part Supplier)
• List the functional dependency in the Part Supplier and identify a candidate key
• For the relation Part Supplier, identify the followings: an insert anomaly, a delete anomaly, and a modification anomaly.
• Draw a relation schema and show the functional dependencies
• Develop a set of 3NF relations from Part Supplier
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