The Database Environment Chapter 5

1
© Prentice Hall, 2002
Chapter 5:Chapter 5:
Logical Database DesignLogical Database Design
and the Relational Modeland the Relational Model
Modern Database Management
6th
Edition
Jeffrey A. Hoffer, Mary B. Prescott, Fred R.
McFadden

2Chapter 5
RelationRelation
 Definition: A relation is a named, two-dimensional table of data
– Table is made up of rows (records), and columns (attribute or field)
 Not all tables qualify as relations
 Requirements:
– Every relation has a unique name.
– Every attribute value is atomic (not multivalued, not composite)
– Every row is unique (can’t have two rows with exactly the same values
for all their fields)
– Attributes (columns) in tables have unique names
– The order of the columns is irrelevant
– The order of the rows is irrelevant
NOTE: all relations are in 1st
Normal form

3Chapter 5
Correspondence with ERCorrespondence with ER
ModelModel
 Relations (tables) correspond with entity types and
with many-to-many relationship types
 Rows correspond with entity instances and with many-
to-many relationship instances
 Columns correspond with attributes
 NOTE: The word relation (in relational database) is
NOT the same same the word relationship (in ER
model)

4Chapter 5
Key FieldsKey Fields
 Keys are special fields that serve two main purposes:
– Primary keys are unique identifiers of the relation in
question. Examples include employee numbers, social
security numbers, etc. This is how we can guarantee that
all rows are unique
– Foreign keys are identifiers that enable a dependent
relation (on the many side of a relationship) to refer to its
parent relation (on the one side of the relationship)
 Keys can be simple (a single field) or composite
(more than one field)
 Keys usually are used as indexes to speed up the
response to user queries (More on this in Ch. 6)

5Chapter 5
Figure 5-3 -- Schema for four relations (Pine Valley Furniture)
Primary Key
Foreign Key
(implements 1:N relationship
between customer and order)
Combined, these are a composite
primary key (uniquely identifies the
order line)…individually they are
foreign keys (implement M:N
relationship between order and
product)

6Chapter 5
Integrity ConstraintsIntegrity Constraints
Domain Constraints
– Allowable values for an attribute. See Table 5-1
Entity Integrity
– No primary key attribute may be null. All primary
key fields MUST have data
Action Assertions
– Business rules. Recall from Ch. 4

7Chapter 5
Integrity ConstraintsIntegrity Constraints
 Referential Integrity – rule that states that any foreign key value (on
the relation of the many side) MUST match a primary key value in the
relation of the one side. (Or the foreign key can be null)
– For example: Delete Rules
 Restrict – don’t allow delete of “parent” side if related rows exist in
“dependent” side
 Cascade – automatically delete “dependent” side rows that correspond with
the “parent” side row to be deleted
 Set-to-Null – set the foreign key in the dependent side to null if deleting
from the parent side  not allowed for weak entities

8Chapter 5
Figure 5-5:
Referential integrity constraints (Pine Valley Furniture)
Referential
integrity
constraints are
drawn via arrows
from dependent to
parent table

9Chapter 5
Transforming EER DiagramsTransforming EER Diagrams
into Relationsinto Relations
Mapping Regular Entities to Relations
1. Simple attributes: E-R attributes map directly
onto the relation
2. Composite attributes: Use only their simple,
component attributes
3. Multi-valued Attribute - Becomes a separate
relation with a foreign key taken from the
superior entity

10Chapter 5
(a) CUSTOMER
entity type with
simple
attributes
Figure 5-8: Mapping a regular entity
(b) CUSTOMER relation

11Chapter 5
(a) CUSTOMER
entity type with
composite
attribute
Figure 5-9: Mapping a composite attribute
(b) CUSTOMER relation with address detail

12Chapter 5
Figure 5-10: Mapping a multivalued attribute
1 – to – many relationship between original entity and new relation
(a)
Multivalued attribute becomes a separate relation with foreign key
(b)

13Chapter 5
Mapping Weak Entities
– Becomes a separate relation with a
foreign key taken from the superior entity
– Primary key composed of:
Partial identifier of weak entity
Primary key of identifying relation (strong
entity)

14Chapter 5
Figure 5-11: Example of mapping a weak entity
(a) Weak entity DEPENDENT

15Chapter 5
Figure 5-11(b) Relations resulting from weak entity
NOTE: the domain constraint
for the foreign key should
NOT allow null value if
DEPENDENT is a weak
entity
Foreign key
Composite primary key

16Chapter 5
Mapping Binary Relationships
– One-to-Many - Primary key on the one side
becomes a foreign key on the many side
– Many-to-Many - Create a new relationnew relation with the
primary keys of the two entities as its primary
key
– One-to-One - Primary key on the mandatory
side becomes a foreign key on the optional side

17Chapter 5
Figure 5-12: Example of mapping a 1:M relationship
(a) Relationship between customers and orders
Note the mandatory one

18Chapter 5
Figure 5-12(b) Mapping the relationship
Again, no null value in the
foreign key…this is because
of the mandatory minimum
cardinality
Foreign key

19Chapter 5
Figure 5-13: Example of mapping an M:N relationship
(a) ER diagram (M:N)
The Supplies relationship will need to become a separate relation

20Chapter 5
Figure 5-13(b) Three resulting relations
New
intersection
relation
Foreign key
Foreign key
Composite primary key

21Chapter 5
Figure 5-14: Mapping a binary 1:1 relationship
(a) Binary 1:1 relationship

22Chapter 5
Figure 5-14(b) Resulting relations

23Chapter 5
Mapping Associative Entities
– Identifier Not Assigned
Default primary key for the association
relation is composed of the primary keys of
the two entities (as in M:N relationship)
– Identifier Assigned
It is natural and familiar to end-users
Default identifier may not be unique

24Chapter 5
Figure 5-15: Mapping an associative entity
(a) Associative entity

25Chapter 5
Figure 5-15(b) Three resulting relations

26Chapter 5
Mapping Unary Relationships
– One-to-Many - Recursive foreign key in the
same relation
– Many-to-Many - Two relations:
One for the entity type
One for an associative relation in which the
primary key has two attributes, both taken
from the primary key of the entity

27Chapter 5
Figure 5-17: Mapping a unary 1:N relationship
(a) EMPLOYEE entity with
Manages relationship
(b) EMPLOYEE
relation with
recursive foreign
key

28Chapter 5
Figure 5-18: Mapping a unary M:N relationship
(a) Bill-of-materials
relationships (M:N)
(b) ITEM and
COMPONENT
relations

29Chapter 5
Mapping Ternary (and n-ary)
Relationships
– One relation for each entity and one
for the associative entity
– Associative entity has foreign keys
to each entity in the relationship

30Chapter 5
Figure 5-19: Mapping a ternary relationship
(a) Ternary relationship with associative entity

31Chapter 5
Figure 5-19(b) Mapping the ternary relationship
Remember that the
primary key MUST be
unique

32Chapter 5
Mapping Supertype/Subtype Relationships
– One relation for supertype and for each subtype
– Supertype attributes (including identifier and
subtype discriminator) go into supertype relation
– Subtype attributes go into each subtype; primary
key of supertype relation also becomes primary
key of subtype relation
– 1:1 relationship established between supertype
and each subtype, with supertype as primary
table

33Chapter 5
Figure 5-20: Supertype/subtype relationships

34Chapter 5
Figure 5-21:
Mapping Supertype/subtype relationships to relations

35Chapter 5
Data NormalizationData Normalization
Primarily a tool to validate and improve a
logical design so that it satisfies certain
constraints that avoid unnecessary
duplication of data
The process of decomposing relations with
anomalies to produce smaller, well-
structured relations

36Chapter 5
Well-Structured RelationsWell-Structured Relations
 A relation that contains minimal data redundancy
and allows users to insert, delete, and update rows
without causing data inconsistencies
 Goal is to avoid anomalies
– Insertion Anomaly – adding new rows forces user to
create duplicate data
– Deletion Anomaly – deleting rows may cause a loss of
data that would be needed for other future rows
– Modification Anomaly – changing data in a row forces
changes to other rows because of duplication
General rule of thumb: a table should not pertain to
more than one entity type

37Chapter 5
Example – Figure 5.2bExample – Figure 5.2b
Question – Is this a relation? Answer – Yes: unique rows and no multivalued
attributes
Question – What’s the primary key? Answer – Composite: Emp_ID,
Course_Title

38Chapter 5
Anomalies in this TableAnomalies in this Table
Insertion – can’t enter a new employee without
having the employee take a class
Deletion – if we remove employee 140, we lose
information about the existence of a Tax Acc class
Modification – giving a salary increase to
employee 100 forces us to update multiple records
Why do these anomalies exist?
Because we’ve combined two themes (entity types)
into one relation. This results in duplication, and an
unnecessary dependency between the entities

39Chapter 5
Functional Dependencies and KeysFunctional Dependencies and Keys
Functional Dependency: The value of one
attribute (the determinant) determines the
value of another attribute
Candidate Key:
– A unique identifier. One of the candidate keys
will become the primary key
 E.g. perhaps there is both credit card number and
SS# in a table…in this case both are candidate keys
– Each non-key field is functionally dependent on
every candidate key

40Chapter 5
5.22 -Steps in
normalization

41Chapter 5
First Normal FormFirst Normal Form
No multivalued attributes
Every attribute value is atomic
Fig. 5-2a is not in 1st
Normal Form
(multivalued attributes)  it is not a
relation
Fig. 5-2b is in 1st
Normal form
All relations are in 1st
Normal Form

42Chapter 5
Second Normal FormSecond Normal Form
1NF plus every non-key attribute is fully
functionally dependent on the ENTIRE
primary key
– Every non-key attribute must be defined by the
entire key, not by only part of the key
– No partial functional dependencies
Fig. 5-2b is NOT in 2nd
Normal Form (see fig
5-23b)

43Chapter 5
Fig 5.23(b) – FunctionalFig 5.23(b) – Functional
Dependencies in EMPLOYEE2Dependencies in EMPLOYEE2
EmpID CourseTitle DateCompletedSalaryDeptNameName
Dependency on entire primary key
Dependency on only part of the key
EmpID, CourseTitle  DateCompleted
EmpID  Name, DeptName, Salary
Therefore, NOT in 2nd
Normal Form!!

44Chapter 5
Getting it into 2Getting it into 2ndnd
Normal FormNormal Form
 See p193 – decomposed into two separate
relations
EmpID SalaryDeptNameName
CourseTitle DateCompletedEmpID
Both are full
functional
dependencies

45Chapter 5
Third Normal FormThird Normal Form
2NF PLUS no transitive dependencies
(one attribute functionally determines a
second, which functionally determines a
third)
Fig. 5-24, 5-25

46Chapter 5
Figure 5-24 -- Relation with transitive dependency
(a) SALES relation with simple data

47Chapter 5
Figure 5-24(b) Relation with transitive dependency
CustID  Name
CustID  Salesperson
CustID  Region
All this is OK
(2nd
NF)
BUT
CustID  Salesperson  Region
Transitive dependency
(not 3rd
NF)

48Chapter 5
Figure 5.25 -- Removing a transitive dependency
(a) Decomposing the SALES relation

49Chapter 5
Figure 5.25(b) Relations in 3NF
Now, there are no transitive dependencies…
Both relations are in 3rd
NF
CustID  Name
CustID  Salesperson
Salesperson  Region

50Chapter 5
Other Normal FormsOther Normal Forms
(from Appendix B)(from Appendix B)
 Boyce-Codd NF
– All determinants are candidate keys…there is no determinant
that is not a unique identifier
 4th
NF
– No multivalued dependencies
 5th
NF
– No “lossless joins”
 Domain-key NF
– The “ultimate” NF…perfect elimination of all possible
anomalies

The Database Environment Chapter 5

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The Database Environment Chapter 5