Chapter 3A Objectives
A Logical View of Data
• Understand
• Relational model
– the relational database model offers a logical view of
data
– the relational model’s basic component: relations
– relations are logical constructs composed of rows
(tuples) and columns (attributes)
– relations are implemented as tables in a relational
DBMS
– View data logically rather than physically
• A Table is a logical construct
– Structural and data independence
• The data is not physically tied to file formats
– Resembles a file conceptually
• Relational database model is easier to understand
than hierarchical and network models
– K.I.S.S. principle
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Tables and Their Characteristics
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Tables and Their Characteristics
• Logical view of relational database is based on
relation
• Entity Set examples
– College DB: students, courses, faculty
•
– Relation thought of as a table
Attributes for Students : StId, StLname, ..
– Airline DB: pilots, routes, aircraft
• Table: two-dimensional structure composed of
rows and columns
• Attributes for AirCraft: AC_regNum,
AC_milesFlown, …
– Persistent representation of logical relation
– Contains group of related entities (entity set)
• Entity Set, Table, and Relation (E.F. Codd’s term)
are used interchangeably
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Tables and Their Characteristics
Tables and Their Characteristics
• Each row is an entity
• Each column is an attribute
– Attributes are characteristics of an entity
– Attribute have naming conventions
• Begin with a character, not a number
• Give names a prefix that represents the table name
• Use the underscore “_” , or camelCase to separate
parts of a name
• don’t use arithmetic operators like dashes in an
name, ie. amount-1
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Determination & Primary Keys
Determination & Primary Keys
• Key’s role is based on determination
• Attribute A determines attribute B if all of the
rows in the table that agree in value for attribute
A also agree in value for attribute B.
• It is possible for functional dependence to be
determined by a range of values.
– If you know the value of attribute A, you can look
up (determine) the value of attribute B
• one or more attributes that determine other
attributes
• Primary key (PK) uniquely identifies any given
entity (row)
• Functional dependence:
– Attribute B is functionally dependent on attribute
A if each value in A determines one and only one
value in B.
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– Later designs might compute this attribute, not
store the classification.
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Choosing a Primary Key
Null Values
• Choosing a Primary Key is a process of finding
attributes based on determination and elimination
of attributes by the following steps.
1. Key attribute
Not permitted in primary key
Means ‘no data entered’
Should be avoided in other attributes
Is used to represent
– An unknown attribute value
– A known, but missing, attribute value
– A “not applicable” condition
• Any attribute that is part of a key
2. Composite key
• Composed of more than one attribute
3. Superkey
• Can create problems in logic and formulas in
minimal DBMS
– when functions such as COUNT, AVERAGE, and
SUM are used
– when relational tables are linked
• Any key that uniquely identifies each row
4. Candidate key
• A superkey without unnecessary attributes
5. Primary key
• The chosen candidate key
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•
•
•
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Relational Schema for CH02_SaleCO
Controlled redundancy
Vendor (v_code, v_contact, v_areaCode, v_phone)
Product(p_code, p_descript, p_price, p_onhand,
v_code)
• Controlled redundancy
– Makes the relational database work
– Tables within the database share common
attributes
• Enables tables to be linked together, unlike
previous DB models which used pointers/links.
– Multiple occurrences of values are not
redundant when required to make the
relationship work
– Redundancy exists only when there is
unnecessary duplication of attribute values
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Relational Diagram for CH02_SaleCO
Relational Database Keys
• Notice that the vend_code attribute name is the
same in both tables.
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Controlled redundancy & Other Keys
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Integrity Rules
• Foreign key (FK)
• Many RDBMs enforce integrity rules
automatically
• Safer to ensure that application design conforms
to entity and referential integrity rules
• Some designers use flags to avoid nulls
– An attribute whose values matches primary key
values in the related table
• Referential integrity
– FKey contains a value that refers to an existing
valid tuple (row) in another relation, or is Null
• Secondary key
– Flags indicate the absence of some value
– Key used strictly for data retrieval purposes
– Often a Super Key that doesn’t become a
Candidate Key & then Primary Key
– Should yield a unique row or few duplicates
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Integrity Rules
Summary of Integrity Rules
• Entity integrity
– Ensures all entities in the set are unique
– Each entity has unique key
• Referential integrity
– Foreign key must have null value or match
primary key values
– Makes it impossible to delete row whose primary
key has mandatory matching foreign key values in
another table
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The Relational Schema & Diagram for
CH03_SaleCo
• Vendor(v_code, v_contact, v_areacode, v_phone)
• Product(p_code, p_descript, p_price, p_onhand,
v_code)
In Customer, Agent_code attributes all have values
which match Agent_code values in Agent.
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Data Redundancy Review
Summary
• Data redundancy leads to data anomalies
• Tables are basic building blocks of a relational
database
• Keys are central to the use of relational tables
• Keys define functional dependencies
– Such anomalies can destroy database effectiveness
• Sometimes, data redundancy is necessary
– Foreign keys create relationships between entities.
• Control data redundancies by using common attributes
shared by tables
• Crucial to exercise data redundancy control
• Exceptions:
–
–
–
–
–
– Allowing for tracking data historically.
• Example: price attribute in product and in line item of invoice,
allowing for the product price to be changed.
–
•
Rarely: Speed of Processing
Good Design deals with conflicting goals of speed,
information requirements, & design elegance.
Superkey
Candidate key
Primary key
Secondary key
Foreign key
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Summary
• Each table row must have a primary key that
uniquely identifies all attributes
• Tables are linked by common attributes through
controlled redundancy
• Foreign key’s are used to link tables
• Secondary key’s provide an optional way of
retrieving a row or a small subset of rows.
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