Logical Design

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Logical Database Design Logical Database Design and the Relational Modeland the Relational Model

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ObjectivesObjectives Logical Database Design with relational Logical Database Design with relational

modelmodel 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 relationsTransform E-R and EER diagrams to relations 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

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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)

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Database DesignDatabase Design Activities of database design transform the Activities of database design transform the

requirements for data storage developed requirements for data storage developed during database analysis into specifications during database analysis into specifications to guide database implementation.to guide database implementation.

Logical design maps conceptual Logical design maps conceptual requirements into data model relating to a requirements into data model relating to a specific DBMSspecific DBMS

Physical design indicates all parameters for Physical design indicates all parameters for data storage. During this phase, a data storage. During this phase, a database is actually defined using Data database is actually defined using Data Definition Language (DDL)Definition Language (DDL)

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The Relational Data ModelThe Relational Data Model The relational model represents data in The relational model represents data in

the form of tables. It is based on the form of tables. It is based on mathematical theory (Relational Algebra). mathematical theory (Relational Algebra). The model consists of following three The model consists of following three components:components:

Data Structure: Data organised in the Data Structure: Data organised in the form of tables with rows and columnsform of tables with rows and columns

Data Manipulation: Powerful operations Data Manipulation: Powerful operations (using SQL) are used to manipulate data(using SQL) are used to manipulate data

Data Integrity: Facilities to specify Data Integrity: Facilities to specify business rules that maintain integritybusiness rules that maintain integrity of of data when they are manipulated.data when they are manipulated.

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

data data Table consists of unnamed rows (records), and named Table consists of unnamed rows (records), and named

columns (attributes or fields). Each row contains data values columns (attributes or fields). Each row contains data values for a single entity.for a single entity.

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 (single valued not multivalued, Every attribute value must be atomic (single valued not multivalued,

not composite)not 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

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Correspondence with E-R Correspondence with E-R ModelModel

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

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)

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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. Properties of primary key are – security numbers, etc. Properties of primary key are – 1) Unique(duplicate values not allowed and 1) Unique(duplicate values not allowed and 2) Not null (value must be specified)2) Not null (value must be specified)

This is how we can guarantee that all rows are uniqueThis is how we can guarantee that 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 its relation (on the many side of a relationship) to refer to 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)

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Other types of keysOther types of keys Candidate Key: The key field which is probable Candidate Key: The key field which is probable

means of identifying a record uniquely (not null means of identifying a record uniquely (not null not duplicate)not duplicate)

Alternate Key: A candidate key which is Alternate Key: A candidate key which is alternative solution to the primary key but not alternative solution to the primary key but not having status of primary key.having status of primary key.

Duplicate Key: Field allowing duplicate valuesDuplicate Key: Field allowing duplicate values Surrogate Key: When there is no primary key, Surrogate Key: When there is no primary key,

extra key field(s) is borrowed from outside (not extra key field(s) is borrowed from outside (not part of database) and used as primary key. e.g. part of database) and used as primary key. e.g. SrNoSrNo

Super Key- Set of one or more attributes which Super Key- Set of one or more attributes which collectively identifies record uniquely. collectively identifies record uniquely. e.g. Primary key + other attribute(s)e.g. Primary key + other attribute(s)

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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)

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Integrity ConstraintsIntegrity Constraints Domain ConstraintsDomain Constraints

Allowable values for an attribute. Values of Allowable values for an attribute. Values of an attribute must be from the same an attribute must be from the same domain.domain.

Entity IntegrityEntity Integrity Every relation should have a primary keyEvery relation should have a primary key No primary key attribute may be null. All No primary key attribute may be null. All

primary key fields primary key fields MUSTMUST have data have data Action AssertionsAction Assertions

Constraints specified by a category of Constraints specified by a category of business rules. business rules.

Referential IntegrityReferential Integrity

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Domain definitions enforce domain integrity constraints

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Integrity ConstraintsIntegrity Constraints In relational model, associations between tables are In relational model, associations between tables are

defined through foreign keys. Referential integrity defined through foreign keys. Referential integrity maintains consistency among the rows of two relations.maintains consistency among the rows of two relations.

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 key value (on the relation of the many side) MUST match a primary key value in the relation of the one match a primary key value in the relation of the one side. (Or the foreign key can be null) side. (Or 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 Set-to-Null – set the foreign key in the dependent side to null if deleting from the parent side if deleting from the parent side not allowed for weak not allowed for weak entitiesentities

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

Referential integrity

constraints are drawn via arrows from dependent to

parent table

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Referential integrity

constraints are implemented with

foreign key to primary key references

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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

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(a) CUSTOMER entity type with simple attributes

Figure 5-8: Mapping a regular entity

(b) CUSTOMER relation

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(a) CUSTOMER entity type with composite attribute

Figure 5-9: Mapping a composite attribute

(b) CUSTOMER relation with address detail

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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)

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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)

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NOTE: the domain constraint for the foreign key should NOT allow null value if DEPENDENT is a weak entity

Foreign key

Composite primary key

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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

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

Note the mandatory one

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Figure 5-12b Mapping the relationship

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

Foreign key

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

The Supplies relationship will need to become a separate relation

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Figure 5-13b Three resulting relations

New intersection

relationForeign key

Foreign key

Composite primary key

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

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Figure 5-14b Resulting relations

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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

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

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Figure 5-16b Three resulting relations

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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

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

(a) EMPLOYEE entity with Manages relationship

(b) EMPLOYEE relation with recursive foreign key

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

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

(b) ITEM and COMPONENT relations

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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

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

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Figure 5-19b Mapping the ternary relationship

Remember that the primary key MUST be

unique

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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

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Figure 5-20: Supertype/subtype relationships

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

These are implemented as one-to-one relationships

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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

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Normalization, its Normalization, its advantagesadvantages Normalization is the formal process of Normalization is the formal process of

building database structures to store data building database structures to store data in an efficient (optimised) manner.in an efficient (optimised) manner.

It provides systematic and scientific way It provides systematic and scientific way for deciding which attributes should be for deciding which attributes should be grouped together in a relation.grouped together in a relation.

It serves as a tool for validating and It serves as a tool for validating and improving logical design of database.improving logical design of database.

Normalization is achieved by designing the Normalization is achieved by designing the database structure in some normal form. database structure in some normal form. Each normal form is application of certain Each normal form is application of certain rules to the database design to refine it.rules to the database design to refine it.

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Advantages of normalization Advantages of normalization contd…contd…

It helps toIt helps to eliminate redundant dataeliminate redundant data reduce or eliminate inconsistent data and reduce or eliminate inconsistent data and

promotes data integritypromotes data integrity make data maintenance and data security make data maintenance and data security

easy easy make efficient use of storage spacemake efficient use of storage space reduce anomalies such as insertion, deletion reduce anomalies such as insertion, deletion

and update anomaliesand update anomalies simplify entities by creating relationships with simplify entities by creating relationships with

one themeone theme build tables that can be easily joined with other build tables that can be easily joined with other

tables to produce informationtables to produce information

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Disadvantages of Disadvantages of normalizationnormalization

Normalization may reduce database Normalization may reduce database performance. To process queries, it is performance. To process queries, it is required to locate tables and then join required to locate tables and then join data to get information. Because of data to get information. Because of normalized database structure it may normalized database structure it may require to refer to many tables.require to refer to many tables.

User has to decide trade-off between User has to decide trade-off between storage space and access time for data. storage space and access time for data. Sometimes it is advisable to have some Sometimes it is advisable to have some redundancy to reduce time in accessing redundancy to reduce time in accessing data due to referencing too many data due to referencing too many tablestables

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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

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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

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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

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Functional Dependencies and Functional Dependencies and KeysKeys

Functional Dependency: The value of Functional Dependency: The value of one attribute (the one attribute (the determinantdeterminant) ) determines the value of another determines the value of another attribute. If attribute B is functionally attribute. If attribute B is functionally dependent on A then A is dependent on A then A is determinant of B and indicated by A determinant of B and indicated by A B B

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 E.g. perhaps there is both credit card number

and SS# in a table…in this case both are and SS# in a table…in this case both are candidate keyscandidate keys

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

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Normal FormsNormal Forms

Normalization is achieved by designing the Normalization is achieved by designing the database structure in some normal form.database structure in some normal form.

Each normal form is application of certain Each normal form is application of certain rules to the database design to refine it.rules to the database design to refine it.

Six normal forms have been formally Six normal forms have been formally defined.defined.

Each normal form builds on the one Each normal form builds on the one before. e.g.- applying third normal form, before. e.g.- applying third normal form, requires design in second normal form requires design in second normal form

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Figure 5.22 -Steps in normalization

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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

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Table with multivalued attributes, not in 1st normal form

Note: this is NOT a relation

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

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

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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

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

fully functionally dependent on the fully functionally dependent on the ENTIRE primary keyENTIRE primary key Every non-key attribute must be defined by Every non-key attribute must be defined by

the entire key, not by only part of the keythe entire key, not by only part of the key No partial functional dependenciesNo partial functional dependencies A relation in 1NF will be in 2NF if A relation in 1NF will be in 2NF if

Primary key consists of only one attribute ORPrimary key consists of only one attribute OR No non-key attributes exist in the relation ORNo non-key attributes exist in the relation OR Every non-key attribute is functionally dependent Every non-key attribute is functionally dependent

on full set of primary key attributeson full set of primary key attributes

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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

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Getting it into Second Normal Getting it into Second Normal FormForm

Partial Dependencies are removed, but there are still transitive dependencies

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

(functional dependencies between tow or (functional dependencies between tow or more non-primary-key attributes)more non-primary-key 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 table and stays as foreign key in the old table

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Getting it into Third Normal Getting it into Third Normal FormForm

Transitive dependencies are removed

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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

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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

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Logical Design