© 2005 by Prentice Hall 1 Chapter 5: Logical Database Design and the Relational Model Modern...

© 2005 by Prentice Hall© 2005 by Prentice Hall 11

Chapter 5:Chapter 5:Logical Database Design Logical Database Design and the Relational Modeland the Relational Model

Modern Database Modern Database ManagementManagement

77thth Edition EditionJeffrey A. Hoffer, Mary B. Prescott, Jeffrey A. Hoffer, Mary B. Prescott,

Fred R. McFaddenFred R. McFadden

22Chapter 5 © 2005 by Prentice Hall© 2005 by Prentice Hall

ObjectivesObjectives Definition of termsDefinition of terms List five properties of relationsList five properties of relations State two properties of candidate keysState two properties of candidate keys Define first, second, and third normal formDefine first, second, and third normal form Describe problems from merging relationsDescribe problems from merging relations Transform E-R and EER diagrams to Transform E-R and EER diagrams to

relationsrelations Create tables with entity and relational Create tables with entity and relational

integrity constraintsintegrity constraints Use normalization to convert anomalous Use normalization to convert anomalous

tables to well-structured relationstables to well-structured relations


Maintenance

Purpose – information requirements structureDeliverable – detailed design specifications

Database activity – logical database design

Project Identification and Selection

Project Initiation and Planning

Analysis

Physical Design

Implementation

Maintenance

Logical DesignLogical Design

The Physical Design Stage of SDLC The Physical Design Stage of SDLC

(Figures 2-4, 2-5 revisited)(Figures 2-4, 2-5 revisited)


RelationRelation Definition: A relation is a named, two-dimensional table Definition: A relation is a named, two-dimensional table

of data of data Table consists of rows (records), and columns (attribute Table consists of rows (records), and columns (attribute

or field)or field) Requirements for a table to qualify as a relation:Requirements for a table to qualify as a relation:

It must have a unique name.It must have a unique name. Every attribute value must be atomic (not multivalued, not Every attribute value must be atomic (not multivalued, not

composite)composite) Every row must be unique (can’t have two rows with exactly the Every row must be unique (can’t have two rows with exactly the

same values for all their fields)same values for all their fields) Attributes (columns) in tables must have unique namesAttributes (columns) in tables must have unique names The order of the columns must be irrelevantThe order of the columns must be irrelevant The order of the rows must be irrelevantThe order of the rows must be irrelevant

NOTE: all NOTE: all relationsrelations are in are in 11stst Normal form Normal form


Correspondence with E-R Correspondence with E-R ModelModel

Relations (tables) correspond with entity types Relations (tables) correspond with entity types and with many-to-many relationship typesand with many-to-many relationship types

Rows correspond with entity instances and Rows correspond with entity instances and with many-to-many relationship instanceswith many-to-many relationship instances

Columns correspond with attributesColumns correspond with attributes

NOTE: The word NOTE: The word relationrelation (in relational (in relational database) is NOT the same as the word database) is NOT the same as the word relationshiprelationship (in E-R model) (in E-R model)


Key FieldsKey Fields

Keys are special fields that serve two main Keys are special fields that serve two main purposes:purposes: Primary keysPrimary keys are are uniqueunique identifiers of the relation in identifiers of the relation in

question. Examples include employee numbers, social question. Examples include employee numbers, social security numbers, etc. security numbers, etc. This is how we can guarantee This is how we can guarantee that all rows are uniquethat all rows are unique

Foreign keysForeign keys are identifiers that enable a are identifiers that enable a dependentdependent relation (on the many side of a relationship) to refer to relation (on the many side of a relationship) to refer to its its parentparent relation (on the one side of the relationship) relation (on the one side of the relationship)

Keys can be Keys can be simplesimple (a single field) or (a single field) or compositecomposite (more than one field)(more than one field)

Keys usually are used as indexes to speed up the Keys usually are used as indexes to speed up the response to user queries (More on this in Ch. 6)response to user queries (More on this in Ch. 6)


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)


Integrity ConstraintsIntegrity Constraints

Domain ConstraintsDomain Constraints Allowable values for an attribute. See Allowable values for an attribute. See

Table 5-1Table 5-1 Entity IntegrityEntity Integrity

No primary key attribute may be null. No primary key attribute may be null. All primary key fields All primary key fields MUSTMUST have data have data

Action AssertionsAction Assertions Business rules. Recall from Ch. 4Business rules. Recall from Ch. 4


Domain definitions enforce domain integrity constraints


Integrity ConstraintsIntegrity Constraints

Referential Integrity – rule that states that any foreign Referential Integrity – rule that states that any foreign key value (on the relation of the many side) MUST match key value (on the relation of the many side) MUST match a primary key value in the relation of the one side. (Or a primary key value in the relation of the one side. (Or the foreign key can be null) the foreign key can be null) For example: Delete RulesFor example: Delete Rules

Restrict – don’t allow delete of “parent” side if related rows Restrict – don’t allow delete of “parent” side if related rows exist in “dependent” sideexist in “dependent” side

Cascade – automatically delete “dependent” side rows that Cascade – automatically delete “dependent” side rows that correspond with the “parent” side row to be deletedcorrespond with the “parent” side row to be deleted

Set-to-Null – set the foreign key in the dependent side to null if Set-to-Null – set the foreign key in the dependent side to null if deleting from the parent side deleting from the parent side not allowed for weak entities not allowed for weak entities


Figure 5-5: Referential integrity constraints (Pine Valley Furniture)

Referential integrity

constraints are drawn via arrows from dependent to

parent table


Referential integrity

constraints are implemented with

foreign key to primary key references


Transforming EER Diagrams Transforming EER Diagrams into Relationsinto Relations

Mapping Regular Entities to Relations Mapping Regular Entities to Relations 1.1. Simple attributes: E-R attributes map Simple attributes: E-R attributes map

directly onto the relationdirectly onto the relation

2.2. Composite attributes: Use only their Composite attributes: Use only their simple, component attributes simple, component attributes

3.3. Multivalued Attribute - Becomes a Multivalued Attribute - Becomes a separate relation with a foreign key separate relation with a foreign key taken from the superior entitytaken from the superior entity


(a) CUSTOMER entity type with simple attributes

Figure 5-8: Mapping a regular entity

(b) CUSTOMER relation


(a) CUSTOMER entity type with composite attribute

Figure 5-9: Mapping a composite attribute

(b) CUSTOMER relation with address detail


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)


Transforming EER Diagrams Transforming EER Diagrams into Relations (cont.)into Relations (cont.)

Mapping Weak EntitiesMapping Weak Entities Becomes a separate relation with a Becomes a separate relation with a

foreign key taken from the superior foreign key taken from the superior entityentity

Primary key composed of:Primary key composed of: Partial identifier of weak entityPartial identifier of weak entity Primary key of identifying relation Primary key of identifying relation

(strong entity)(strong 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



Mapping Binary RelationshipsMapping Binary Relationships One-to-Many - Primary key on the one side One-to-Many - Primary key on the one side

becomes a foreign key on the many sidebecomes a foreign key on the many side Many-to-Many - Create a Many-to-Many - Create a new relationnew relation

with the primary keys of the two entities with the primary keys of the two entities as its primary keyas its primary key

One-to-One - Primary key on the One-to-One - Primary key on the mandatory side becomes a foreign key on mandatory side becomes a foreign key on the optional sidethe optional side


Figure 5-12a: Example of mapping a 1:M relationshipRelationship between customers and orders

Note the mandatory one


Figure 5-12b Mapping the relationship

Again, no null value in the foreign key…this is because of the mandatory minimum cardinality

Foreign key


Figure 5-13a: Example of mapping an M:N relationshipE-R diagram (M:N)

The Supplies relationship will need to become a separate relation


Figure 5-13b Three resulting relations

New intersection

relationForeign key

Foreign key

Composite primary key


Figure 5-14a: Mapping a binary 1:1 relationshipIn_charge relationship


Figure 5-14b Resulting relations



Mapping Associative EntitiesMapping Associative Entities Identifier Not Assigned Identifier Not Assigned

Default primary key for the Default primary key for the association relation is composed of association relation is composed of the primary keys of the two entities the primary keys of the two entities (as in M:N relationship)(as in M:N relationship)

Identifier Assigned Identifier Assigned It is natural and familiar to end-usersIt is natural and familiar to end-users Default identifier may not be uniqueDefault identifier may not be unique


Figure 5-16a: Mapping an associative entity with an identifierAssociative entity


Figure 5-16b Three resulting relations



Mapping Unary RelationshipsMapping Unary Relationships One-to-Many - Recursive foreign key in One-to-Many - Recursive foreign key in

the same relationthe same relation Many-to-Many - Two relations:Many-to-Many - Two relations:

One for the entity typeOne for the entity type One for an associative relation in which One for an associative relation in which

the primary key has two attributes, the primary key has two attributes, both taken from the primary key of the both taken from the primary key of the entityentity


Figure 5-17: Mapping a unary 1:N relationship

(a) EMPLOYEE entity with Manages relationship

(b) EMPLOYEE relation with recursive foreign key


Figure 5-18: Mapping a unary M:N relationship

(a) Bill-of-materials relationships (M:N)

(b) ITEM and COMPONENT relations



Mapping Ternary (and n-ary) Mapping Ternary (and n-ary) RelationshipsRelationships One relation for each entity and One relation for each entity and

one for the associative entityone for the associative entity Associative entity has foreign keys Associative entity has foreign keys

to each entity in the relationshipto each entity in the relationship


Figure 5-19a: Mapping a ternary relationshipTernary relationship with associative entity


Figure 5-19b Mapping the ternary relationship

Remember that the primary key MUST be

unique


Transforming EER Transforming EER Diagrams into Relations Diagrams into Relations

(cont.)(cont.)Mapping Supertype/Subtype RelationshipsMapping Supertype/Subtype Relationships

One relation for supertype and for each subtypeOne relation for supertype and for each subtype Supertype attributes (including identifier and Supertype attributes (including identifier and

subtype discriminator) go into supertype relationsubtype discriminator) go into supertype relation Subtype attributes go into each subtype; primary Subtype attributes go into each subtype; primary

key of supertype relation also becomes primary key of supertype relation also becomes primary key of subtype relationkey of subtype relation

1:1 relationship established between supertype 1:1 relationship established between supertype and each subtype, with supertype as primary and each subtype, with supertype as primary tabletable


Figure 5-20: Supertype/subtype relationships


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

These are implemented as one-to-one relationships


Data NormalizationData Normalization Primarily a tool to validate and Primarily a tool to validate and

improve a logical design so that it improve a logical design so that it satisfies certain constraints that satisfies certain constraints that avoid unnecessary avoid unnecessary duplication of dataduplication of data

The process of decomposing The process of decomposing relations with anomalies to produce relations with anomalies to produce smaller, smaller, well-structuredwell-structured relationsrelations


Well-Structured RelationsWell-Structured Relations A relation that contains minimal data redundancy A relation that contains minimal data redundancy

and allows users to insert, delete, and update and allows users to insert, delete, and update rows without causing data inconsistenciesrows without causing data inconsistencies

Goal is to avoid anomaliesGoal is to avoid anomalies Insertion AnomalyInsertion Anomaly – adding new rows forces user to – adding new rows forces user to

create duplicate datacreate duplicate data Deletion AnomalyDeletion Anomaly – deleting rows may cause a loss of – deleting rows may cause a loss of

data that would be needed for other future rowsdata that would be needed for other future rows Modification AnomalyModification Anomaly – changing data in a row – changing data in a row

forces changes to other rows because of duplicationforces changes to other rows because of duplication

General rule of thumb: a table should not pertain to more than one entity type


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


Anomalies in this TableAnomalies in this Table InsertionInsertion – can’t enter a new employee without – can’t enter a new employee without

having the employee take a classhaving the employee take a class DeletionDeletion – if we remove employee 140, we lose – if we remove employee 140, we lose

information about the existence of a Tax Acc information about the existence of a Tax Acc classclass

ModificationModification – giving a salary increase to – giving a salary increase to employee 100 forces us to update multiple employee 100 forces us to update multiple recordsrecordsWhy do these anomalies exist?

Because there are two themes (entity types) into one relation. This results in duplication, and an unnecessary dependency between the entities


Functional Dependencies and Functional Dependencies and KeysKeys

Functional Dependency: The value of one Functional Dependency: The value of one attribute (the attribute (the determinantdeterminant) determines ) determines the value of another attributethe value of another attribute

Candidate Key:Candidate Key: A unique identifier. One of the candidate A unique identifier. One of the candidate

keys will become the primary keykeys will become the primary key E.g. perhaps there is both credit card number and E.g. perhaps there is both credit card number and

SS# in a table…in this case both are candidate SS# in a table…in this case both are candidate keyskeys

Each non-key field is functionally dependent Each non-key field is functionally dependent on every candidate keyon every candidate key


Figure 5.22 -Steps in normalization


First Normal FormFirst Normal Form No multivalued attributesNo multivalued attributes Every attribute value is atomicEvery attribute value is atomic Fig. 5-25 Fig. 5-25 is notis not in 1 in 1stst Normal Form Normal Form

(multivalued attributes) (multivalued attributes) it is not a it is not a relationrelation

Fig. 5-26 Fig. 5-26 isis in 1 in 1stst Normal form Normal form All relationsAll relations are in 1 are in 1stst Normal Form Normal Form


Table with multivalued attributes, not in 1st normal form

Note: this is NOT a relation


Table with no multivalued attributes and unique rows, in 1st normal form

Note: this is relation, but not a well-structured one


Anomalies in this TableAnomalies in this Table InsertionInsertion – if new product is ordered for – if new product is ordered for

order 1007 of existing customer, customer order 1007 of existing customer, customer data must be re-entered, causing duplicationdata must be re-entered, causing duplication

DeletionDeletion – if we delete the Dining Table from – if we delete the Dining Table from Order 1006, we lose information concerning Order 1006, we lose information concerning this item's finish and pricethis item's finish and price

UpdateUpdate – changing the price of product ID 4 – changing the price of product ID 4 requires update in several recordsrequires update in several records

Why do these anomalies exist? Because there are multiple themes (entity types) into one relation. This results in duplication, and an unnecessary dependency between the entities


Second Normal FormSecond Normal Form 1NF PLUS 1NF PLUS every non-key every non-key

attribute is fully functionally attribute is fully functionally dependent on the ENTIRE dependent on the ENTIRE primary keyprimary key Every non-key attribute must be Every non-key attribute must be

defined by the entire key, not by only defined by the entire key, not by only part of the keypart of the key

No partial functional dependenciesNo partial functional dependencies


Order_ID Order_Date, Customer_ID, Customer_Name, Customer_Address

Therefore, NOT in 2nd Normal Form

Customer_ID Customer_Name, Customer_Address

Product_ID Product_Description, Product_Finish, Unit_Price

Order_ID, Product_ID Order_Quantity


Getting it into Second Normal Getting it into Second Normal FormForm

Partial Dependencies are removed, but there are still transitive dependencies


Third Normal FormThird Normal Form 2NF PLUS 2NF PLUS no transitive dependenciesno transitive dependencies

(functional dependencies on non-primary-key (functional dependencies on non-primary-key attributes)attributes)

Note: this is called transitive, because the Note: this is called transitive, because the primary key is a determinant for another primary key is a determinant for another attribute, which in turn is a determinant for a attribute, which in turn is a determinant for a thirdthird

Solution: non-key determinant with transitive Solution: non-key determinant with transitive dependencies go into a new table; non-key dependencies go into a new table; non-key determinant becomes primary key in the new determinant becomes primary key in the new table and stays as foreign key in the old table and stays as foreign key in the old table table


Getting it into Third Normal Getting it into Third Normal FormForm

Transitive dependencies are removed


Merging RelationsMerging Relations View Integration – Combining entities from View Integration – Combining entities from

multiple ER models into common relationsmultiple ER models into common relations Issues to watch out for when merging entities Issues to watch out for when merging entities

from different ER models:from different ER models: Synonyms – two or more attributes with different Synonyms – two or more attributes with different

names but same meaningnames but same meaning Homonyms – attributes with same name but Homonyms – attributes with same name but

different meaningsdifferent meanings Transitive dependencies – even if relations are in Transitive dependencies – even if relations are in

3NF prior to merging, they may not be after merging3NF prior to merging, they may not be after merging Supertype/subtype relationships – may be hidden Supertype/subtype relationships – may be hidden

prior to mergingprior to merging


Enterprise KeysEnterprise Keys

Primary keys that are unique in the whole Primary keys that are unique in the whole database, not just within a single relationdatabase, not just within a single relation

Corresponds with the concept of an object Corresponds with the concept of an object ID in object-oriented systemsID in object-oriented systems

© 2005 by Prentice Hall 1 Chapter 5: Logical Database Design and the Relational Model Modern...

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