1. Lecture 2 The Relational Model
Updated 19/9/2016
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Objectives
In this chapter, students will learn about the fundamental of a relational model
Domains
Relations
Attributes
Relationships
Keys
Integrity Rules
Relational operations
Data Dictionary
Indexes
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Data Models
Model is a collection of tools: diagrams and a set of vocabularies
A data model provides description of the database structure: data, relationships, constraints, operations and meanings
Examples: entity relationship model, object- oriented model, object-relational model and large- scale data models
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A Relational Model
By far the most widely used model for database design
Built on the algebra
Consist of a collection of relations
View data logically rather than physically
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Domains
A set of scalar values from which attribute values are drawn. Scalar values are non-decomposable (atomic) values.
Values permitted for each attribute
Domains are conceptual, not store in database
Domains subset containing actual attributes are stored
Domains are analogous to data types in programming languages in C++, Java etc
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Relations (1)
Logical view of relational database is based on relation
Relation thought of as a table (also entity)
Table: two-dimensional structure composed of rows and columns
Persistent representation of logical relation
Contains group of related entities (relation also known as entity set)
Examples: Students, Courses …
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Relations (2)
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Relations (3)
Various types of relations
Base relations
View (virtual relations)
Snapshots (read-only)
Query results
A query always results in a new relations
Temporary relations
Destroyed at some point in time
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Attributes (1)
A very important relation properties
An attribute has different permitted values from a domain
Examples: Student Id, course name …
How to determine if an object (entity) is a relation or an attributes?
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Attributes (2)
Null Values
Absence of any data value that could represent:
An unknown attribute value
A known, but missing, attribute value
A inapplicable condition
Causes
Can create problems when functions such as COUNT, AVERAGE, and SUM are used
Can create logical problems when relational tables are linked
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Keys (1)
Consist of one or more attributes that determine other attributes
Used to:
Ensure that each row in a table is uniquely identifiable
Establish relationships among tables and to ensure the integrity of the data
Primary key (PK): Attribute or combination of attributes that uniquely identifies any given row
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Keys (2)
Determination
State in which knowing the value of one attribute makes it possible to determine the value of another
Is the basis for establishing the role of a key
Based on the relationships among the attributes
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Keys (3)
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Quiz 1
A _____ key is any key that uniquely identifies each row
A. super
B. special
C. selective
D. candidate
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Relationships (1)
1:M relationship
Relational modeling ideal
Should be the norm in any relational database design
1:1 relationship
Should be rare in any relational database design
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Relationships (2)
M:N relationships
Cannot be implemented as such in the relational model
M:N relationships can be changed into 1:M relationships
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The 1:M Relationship
Relational database norm
Found in any database environment
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The 1:1 Relationship
One entity related to only one other entity, and vice versa
Sometimes means that entity components were not defined properly
Could indicate that two entities actually belong in the same table
Certain conditions absolutely require their use
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The M:N Relationship
Implemented by breaking it up to produce a set of 1:M relationships
Avoid problems inherent to M:N relationship by creating a composite entity
Includes as foreign keys the primary keys of tables to be linked
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Integrity Rules
Many RDBMs enforce integrity rules automatically
Safer to ensure that application design conforms to entity and referential integrity rules
Null value breaks integrity
Designers use flags to avoid nulls
Flags indicate absence of some value
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32. Integrity Rules – Entity Integrity
Description
Requirement
All primary key entries are unique, and no part of a primary key may be null
Purpose
Each row will have a unique identity, and foreign key values can properly reference primary key values
Example
No invoice can have a duplicate number, nor it can be null
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33. Integrity Rules - Referential Integrity
Entity Integrity
Description
Requirement
A foreign key may have either a null entry or a entry that matches a primary key value in a table to which it is related
Purpose
It is possible for an attribute not to have a corresponding value but it is impossible to have an invalid entry
It is impossible to delete row in a table whose primary keys has mandatory matching foreign key values in another table
Example
It is impossible to have invalid sales representative number
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Quiz 2
Referential ____ means if the foreign key contains a value, that value refers to an existing valid tuple (row) in another relation.
A. integrity
B. uniqueness
C. direction
D. relations
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Relational Algebra
Defines the theoretical way of manipulating table contents using relational operators
Relvar: Variable that holds a relation
Heading contains the names of the attributes and the body contains the relation
Relational operators have the property of closure
Closure: Use of relational algebra operators on existing relations produces new relations
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37. Relational Set Operators (1)
Relational algebra
SELECT
UNION
PROJECT
DIFFERENCE
JOIN
PRODUCT
INTERSECT
DIVIDE
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38. Relational Set Operators (2)
Unary operator that yields a horizontal subset of a table
Select (Restrict)
Unary operator that yields a vertical subset of a table
Project
Combines all rows from two tables, excluding duplicate rows
Union-compatible: Tables share the same number of columns, and their corresponding columns share compatible domains
Union
Yields only the rows that appear in both tables
Tables must be union-compatible to yield valid results
Intersect
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Select
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Project
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Union
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Intersect
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43. Relational Set Operators (3)
Difference
Yields all rows in one table that are not found in the other table
Tables must be union-compatible to yield valid results
Product
Yields all possible pairs of rows from two tables
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Difference
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Product
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46. Relational Set Operators (4)
Join
Allows information to be intelligently combined from two or more tables
Divide
Uses one 2-column table as the dividend and one single- column table as the divisor
Output is a single column that contains all values from the second column of the dividend that are associated with every row in the divisor
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Types of Joins
Natural join
Links tables by selecting rows with common values in common attributes (join columns)
Equijoin
Links tables on the basis of an equality condition that compares specified columns
Theta join
Any other comparison operator is used
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49. Relational Set Operators (5)
Inner join
Only returns matched records from the tables that are being joined
Outer join
Matched pairs are retained, and any unmatched values in other table are left null
Left outer join
Right outer join
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Quiz 3
To be considered minimally relational, the DBMS must support the key relational operators ____, PROJECT, and JOIN.
A. INTERSECT
B. UNION
C. DIFFERENCE
D. SELECT
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51. The Data Dictionary and System Catalog
Provides detailed accounting of all tables found within the user/designer-created database
Contains (at least) all the attribute names and characteristics for each table in the system
Contains metadata: data about data
System catalog
Contains metadata
Detailed system data dictionary that describes all objects within the database
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53. The Data Dictionary and System Catalog (cont’d.)
Homonym
Indicates the use of the same name to label different attributes
C_Name for Car and Customer name
Synonym
Opposite of a homonym
Indicates the use of different names to describe the same attribute
CUSTOMER vs C_Name
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Indexes
Orderly arrangement to logically access rows in a table
Index key
Index’s reference point
Points to data location identified by the key
Unique index
Index in which the index key can have only one pointer value (row) associated with it
Each index is associated with only one table
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Questions?
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