Transcript
Database Design – Lecture 8
Normalization
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Lecture Objectives
Review basic rules for converting Entities to Tables
Review how to address Relationships between entities
Overview of Normalization
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Converting ERD into Relational Database Model
Entities to Tables1. Create a separate table for each entity and
consider all the attributes you defined in ERD as the table columns
2. Define a unique identifier as the table PK. Could be an existing attribute that can be used for PK otherwise create a generic PK.
3. Do not include derived attributes in the tables.
4. Ensure all attributes are atomic.5. Create a new table to support multi-valued
attributes.
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Converting ERD into Relational Database Model
Relationships1. For 1:M connectivity, include the PK of the
’1’ as an FK attribute in the ‘M’ table. Add additional attributes to the ‘M’ table, as required.
2. For M:N connectivity, create a bridge table. Include the PKs from both entities as a composite PK in the bridge table. These attributes will also be FKs in the bridge table. Add additional attributes to the bridge table, as required.
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Converting ERD into Relational Database Model
Relationships3. For Strong-Weak relationship, include PK of
strong entity as part of the PK of the weak entity, thus giving it a composite primary key. It will also be an FK in the weak entity.
4. For Supertype/Subtype relationships, create a new entity. Attributes unique to the subtype are moved to the new table and are removed from the supertype entity. Both entities will have the same primary keys.
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Converting ERD into Relational Database Model
Relationships5. For recursive relationships (1:1, 1:M), add
additional attributes as required. For M:N recursive relationship, create a new entity with a composite primary key. Create a primary key for the new entity and make a composite primary key using it and the primary key from the ‘1’ entity. The primary key from the ‘1’ will also be a FK in the new entity.
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Normalization
Normalization can be used in two ways: As a process to validate the table structures
created through the conversion of entity relationship diagrams to relational tables
As a process to create entities from table structures created by user views
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Normalization
Normalization is a process for assigning attributes to entities Reduces data redundancies Helps eliminate data anomalies Produces controlled redundancies to link
tables Works through a series of stages called
normal forms Works with views of data
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Database Tables and Normalization
Normalization stages 1NF - First normal form
Eliminate repeating groups Identify PK result is functional dependency
2NF - Second normal form Eliminate partial dependencies
3NF - Third normal form Eliminate transitive dependencies
BCNF – Boyce-Codd normal form Eliminate dependencies whereby a non-key
attribute can identify a key attribute.
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Need for Normalization
Figure 5.2
possible primary keys?
• Table entries have data inconsistencies (note blank entries)• Table displays data anomalies:
• Update - Modifying JOB_CLASS• Insertion - New employee must be assigned project• Deletion – If an employee is deleted, other vital data lost
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Conversion to 1NF
Repeating groups must be eliminated Proper primary key developed
Uniquely identifies attribute values - rows (functional dependency)
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Data Organization: 1NF
Figure 5.2
Eliminate repeating groups – populate every cell of the ‘table’.
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Conversion to 1NF
Identify dependencies Desirable dependencies based on primary
key Less desirable dependencies
Partial (later) based on part of composite primary key
Transitive (later) one nonprime attribute depends on another
nonprime attribute
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Dependency Diagram (1NF)
A B C d e f
First Normal Form: identify dependencies
Transitive Dependency (non-key attribute depends on another non-key attribute)
Note: Capital letters refer to primary key, lower case letters refer to attributes.
Partial Dependency: attributes dependent on part of the primary key
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Dependency Diagram (1NF)
A B C d e f
First Normal Form: identify all dependencies
Transitive Dependency (non-key attribute depends on another non-key attribute)
Functional Dependency: A, B, C, d, e, fPartial Dependency: A, d, eTransitive Dependency: d, f
Partial Dependency: attributes dependent on part of the primary key
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Data Organization: 1NF
Figure 5.2
Eliminate repeating groups – populate every cell of the ‘table’.
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Dependency Diagram (1NF)
PROJ_NUM
EMP_NUM
PROJ_NAME
EMP_NAME
JOB_CLASS
CHG_HOUR
HOURS
Functional Dependency: each attribute is uniquelyidentified by, or is dependent on the primary key.
Primary key is a composite primary key and is made up of PROJ_NUM and EMP_NUM
EMPLOYEE_PROJECT (PROJ_NUM (pk), EMP_NUM (pk), PROJ_NAME, EMP_NAME, JOB_CLASS, CHG_HOUR, HOURS)
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1NF Summarized
All key attributes defined No repeating groups in table All attributes dependent on primary key
(functional dependency)
EMPLOYEE_PROJECT (PROJ_NUM (pk), EMP_NUM (pk), PROJ_NAME, EMP_NAME, JOB_CLASS, CHG_HOUR, HOURS)
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Dependency Diagram Continued
PROJ_NUM
EMP_NUM
PROJ_NAME
EMP_NAME
JOB_CLASS
CHG_HOUR
HOURS
1NF 2NF
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Conversion to 2NF
Start with 1NF format: Write each key component on separate line Write original key on last line Each component is new table Write dependent attributes after each key
PROJECT (PROJ_NUM (pk), PROJ_NAME)
EMPLOYEE (EMP_NUM (pk), EMP_NAME, JOB_CLASS, CHG_HOUR)
EMPLOYEE_PROJECT (PROJ_NUM (pk, fk), EMP_NUM (pk, fk), HOURS)
Attribute of hours is dependent on composite primary key
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2NF Summarized
In 1NF Includes no partial dependencies
Attributes dependent on a portion of primary key
Still possible to exhibit transitive dependency Attributes may be functionally dependent
on non-key attributes
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Dependency Diagram Continued
PROJ_NUM
EMP_NUM
PROJ_NAME
EMP_NAME
JOB_CLASS
CHG_HOUR
HOURS
1NF 2NF 3NF
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Conversion to 3NF Create separate tables to eliminate transitive
functional dependencies Identify any additional attributes needed in new table
JOB (JOB_CLASS (pk), JOB_DESCRIPTION, CHG_HOUR)
PROJECT (PROJ_NUM (pk), PROJ_NAME)
EMPLOYEE (EMP_NUM (pk), EMP_NAME, JOB_CLASS (fk))
EMPLOYEE_PROJECT (PROJ_NUM (pk, fk), EMP_NUM (pk, fk), HOURS)
New attribute
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3NF Summarized
In 2NF Contains no transitive
dependencies
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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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Normalization Steps
1NF All key attributes are defined No repeating groups All attributes are functionally dependent on
the primary key
2NF Table is in 1NF No partial dependencies
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Normalization Steps
3NF Table is in 2NF No transitive dependencies New PKs identified where appropriate
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