Transcript File - Malak AL

Database Systems:
Design, Implementation, and
Management
Ninth Edition
Chapter 4
The Relational Model Charactristics
Objectives
• In this chapter, students will learn:
– That the relational database model offers a
logical view of data
– About the relational model’s basic component:
relations
– That relations are logical constructs composed
of rows (tuples) and columns (attributes)
– That relations are implemented as tables in a
relational DBMS
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Objectives (cont’d.)
– About relational database operators, the data
dictionary, and the system catalog
– How data redundancy is handled in the
relational database model
– Why indexing is important
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A Logical View of Data
• Relational model
– View data logically rather than physically
• Table
– Structural and data independence
– Resembles a file conceptually
• Relational database model is easier to
understand than hierarchical and network
models
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Tables and Their Characteristics
• Logical view of relational database is based on
relation
– Relation thought of as a table
• Table: two-dimensional structure composed of
rows and columns
– Persistent representation of logical relation
• Contains group of related entity occurrences
(entity set)
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Keys
• Each row in a table must be uniquely
identifiable
• Key is one or more attributes that determine
other attributes
• Key’s role is based on determination
– If you know the value of attribute A, you can
determine the value of attribute B
• Functional dependence
– Attribute B is functionally dependent on A if
each value in column A determines one and
only one value in column B
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Keys (cont’d.)
• Composite key
– Any determinant Composed of more than one
attribute
• Key attribute
– Any attribute that is part of a key
• Candidate key
– Any determinant that determines ALL other
attributes in the relation ( table )
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Keys (cont’d.)
• Nulls
– No data entry at all .It does not mean zero or space
– Not permitted in primary key to maintain entity
integrity
– Should be avoided in other attributes
– If they are used improperly they Can represent:
• An unknown attribute value
• A known, but missing, attribute value
• A “not applicable” condition
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Keys (cont’d.)
• Nulls (cont’d.)
– 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 (cont’d.)
• Controlled redundancy makes the relational
database work
• Tables within the database share common
attributes that enables them to be linked
together
– Multiple occurrences of values not redundant
when required to make the relationship work
– Redundancy exists only when there is
unnecessary duplication of attribute values
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Keys (cont’d.)
• Foreign key (FK)
– An attribute whose values match primary key
values in the related table
• Secondary key
– Key used strictly for data retrieval purposes
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Integrity Rules
• Many RDBMs enforce integrity rules
automatically but t is much safer to ensure that
application design conforms to integrity rules
We have 2 integrity rules :
1) Entity integrity :
ALL primary key values must be unique and does not
contain null values
2) Referential integrity:
FK values should refer to an existing valid tuple ( raw ) in
another relation ( table )
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• Designers use flags to avoid nulls
Flags indicate absence of some value
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Relational Set Operators
• Relational algebra
– Defines theoretical way of manipulating table
contents using relational operators
– Closure :the use of relational algebra operators
on existing relations produces new relations:
• SELECT
• PROJECT
• DIFFERENCE
• JOIN
• UNION
• INTERSECT
• PRODUCT
• DIVIDE
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Give an example of using UNION /INTERSECT/ DIFFERENCE
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Relational Set Operators (cont’d.)
• Natural Join
– Links tables by selecting rows with common
values in common attribute(s)
• Equijoin
– Links tables on the basis of an equality
condition that compares specified columns
• Theta join
– Any other comparison operator is used
• Outer join
– Matched pairs are retained, and any
unmatched values in other table are left null
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ROOM #
TEACHER
411
Malak
411
Anwar
410
Malak
410
Amal
410
Masheal
Database Systems, 9th Edition
Who is using Room # 410 and 411
together this semester?
ROOM #
TEACHER
410
Malak
411
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The Data Dictionary and System Catalog
• Data dictionary
– Provides detailed description 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
– Is a system-created database whose tables store the
user/designer-created database characteristics and contents.
therefore, the system catalog tables can be queried just like any
user/designer-created table
– Automatically produces database documentation which allows
the RDBMS to check for and eliminate homonyms & synonyms 32
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Relationships within the Relational
Database
• 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 within the Relational
Database (cont’d.)
• M:N relationships
– Cannot be implemented as such in the relational
model
– M:N relationships can be changed into two1:M
relationships
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The 1:M Relationship
• Relational database norm
• Found in any database environment
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Another EX: page 77 / 123
Each course can generate many classes
Each class refers to only one course
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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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Optional
Look at the figure 3.23 /Page 79 /125
Another relationship: department employs professor
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The M:N Relationship
• M:N relationship is not supported directly in the
relational model
• Can be implemented by creating a new entity
(composite entity/bridge entity/associative
entity) ---linking table witch include the
primary keys of the tables that are to be linked
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Look at figure 3.2 page 81
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Or you can create
new primary key
as ENROLL_LINE
What is the
appropriate data
type for this key?
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Data Redundancy Revisited
• Data redundancy leads to data anomalies
– Can destroy the effectiveness of the database
• Foreign keys
– Control data redundancies by using common
attributes shared by tables
– minimize data redundancies
• Sometimes, data redundancy is necessary
Look at figure 3.30 /page 85
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Information
requirements
Processing speed
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Design
elegance
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Indexes
• Orderly arrangement to logically access rows in a
table
• Is composed of an index key & set of pointers
• 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
• A table can have many indexes, but each index is
associated with only one table
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Codd’s Relational Database Rules
• In 1985, Codd published a list of 12 rules to
define a relational database system
– Products marketed as “relational” that did not
meet minimum relational standards
• Even dominant database vendors do not fully
support all 12 rules
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Summary
• Tables are basic building blocks of a
relational database
• Keys are central to the use of relational tables
• Keys define functional dependencies
–
–
–
–
–
Superkey
Candidate key
Primary key
Secondary key
Foreign key
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Summary (cont’d.)
• Each table row must have a primary key that
uniquely identifies all attributes
• Tables are linked by common attributes
• The relational model supports relational algebra
functions
– SELECT, PROJECT, JOIN, INTERSECT
UNION, DIFFERENCE, PRODUCT, DIVIDE
• Good design begins by identifying entities,
attributes, and relationships
– 1:1, 1:M, M:N
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Q8 ,9,10
Natural JOIN
STU_CODE
128569
512272
531235
553427
PROF_CODE
2
4
2
1
STU_CODE
128569
512272
531235
553427
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PROFESSOR
PROF_CODE
2
4
2
1
PROF_CODE
2
4
2
1
Chen ERD (generated with PowerPoint)
DEPT_CODE
6
4
6
2
DEPT_CODE
6
4
6
2
M
advises
STUDENT
Crow’s Foot ERD (generated with PowerPoint)
PROFESSOR
advises
STUDENT
Chen ERD (generated with Visio Professional)
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Q11,12,13,14
BOOTH_PRODUCT
BOOTH_PRICE
Chips
1.5
Cola
1.25
Energy Drink
2
Chips
1.25
Chocolate Bar
BOOTH_PRODUCT
1
BOOTH_PRICE
Energy Drink
MACHINE_PRODUCT
2
MACHINE_PRICE
Chips
Chocolate Bar
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1.25
1
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