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CMIS 450: Database DesignCMIS 450: Database DesignDr. Bijoy BordoloiDr. Bijoy Bordoloi

Transforming E/R Diagrams to Transforming E/R Diagrams to RelationsRelations

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Represent EntitiesRepresent Entities

• Each entity-type in E/R Diagram becomes a Each entity-type in E/R Diagram becomes a Relation (Table)Relation (Table)

• The identifier of the entity-type becomes the The identifier of the entity-type becomes the PK of the corresponding relation (provided PK of the corresponding relation (provided the PK meets all the necessary criteria the PK meets all the necessary criteria discussed earlier)discussed earlier)

• Each non-key attribute of the entity-type Each non-key attribute of the entity-type becomes a non-key attribute of the relationbecomes a non-key attribute of the relation

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Represent EntitiesRepresent Entities

• QuestionQuestion– What happens if you allow (non-identifying) What happens if you allow (non-identifying)

multi-valued attributes in an entity-type? Can multi-valued attributes in an entity-type? Can the identifier of an entity-type then also be the the identifier of an entity-type then also be the PK of the corresponding relation? PK of the corresponding relation?

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Attributes CardinalityAttributes Cardinality

• Describes association between attribute and Describes association between attribute and owner entityowner entity

• Single-valued attribute has at most one value Single-valued attribute has at most one value for each entity instancefor each entity instance

• Multi-valued attribute has many values for Multi-valued attribute has many values for some entity instancessome entity instances

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Attributes CardinalityAttributes Cardinality

EMPLOYEE

EMP-ID SS-NUM SEX EXTENSION**

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Multi-valued AttributesMulti-valued Attributes

Either redefine the Primary Key* or make a Either redefine the Primary Key* or make a separate relation with a foreign key taken separate relation with a foreign key taken from the superior entityfrom the superior entity

* Refer to class notes on Relational Model – * Refer to class notes on Relational Model – Primary KeyPrimary Key

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Multi-valued AttributesMulti-valued Attributes

• Multi-valued attributes become new Multi-valued attributes become new characteristic tablescharacteristic tables

EMPLOYEE

EMP#

E-NAME DATETITLE

M M

N

N

EMPLOYEE HISTORY

EMP# E-NAME

EMP#TITLE DATE

1

11

NN

N

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Multi-valued AttributesMulti-valued Attributes

• Characteristic table is necessary because relational model is normalizedCharacteristic table is necessary because relational model is normalized

EMP# E-NAME TITLE

HISTORY

DATE EMP# E-NAME

EMPLOYEENOT IN 2NF

MA TABLE

EMP # DATE TITLE

E1 3-15-85 JANITORE1 3-16-85 TECHNICIANE1 3-17-85 MANAGERE2 2-23-87 EMPLOYERE2 4-19-88 DOGCATCHER

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Transforming RelationshipsTransforming Relationships

Mapping Binary RelationshipsMapping Binary Relationships– One-to-Many – and One-to-One relationships One-to-Many – and One-to-One relationships

become become foreign keys. foreign keys. – Many-to-Many - Create a Many-to-Many - Create a new relationnew relation with the with the

primary keys of the two entities as its primary primary keys of the two entities as its primary keykey

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Cont’dCont’d

• In N-1 relationship, always place foreign In N-1 relationship, always place foreign key at ‘N’ sidekey at ‘N’ side

• In 1-1 relationship, usually place foreign In 1-1 relationship, usually place foreign key in smaller table (results in fewer null key in smaller table (results in fewer null values)values)

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Representing a (1:N) RelationshipRepresenting a (1:N) Relationship

CUSTOMER

ORDER

Places

CUSTOMER NO.

NAME CITY STATE ZIP

ADDRESS

DISCOUNT

ORDER DATE

PROMISED DATE

ORDER NO.

a) E-R Diagram

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Representing a (1:N) RelationshipRepresenting a (1:N) Relationship

CUSTOMER NO. NAME ADDRESS CITY STATE ZIP DISCOUNT1273 Contemporaries 123 Oak St. Austin, TX 38405 5%6390 Casual Corner 18 Hoosier Dr. Bloomington, IN 45821 3%…

CUSTOMER

ORDER NO. ORDER DATE PROMISED DATE CUSTOMER NO.57194 3/15/9X 3/28/9X 639063725 3/17/9X 4/01/9X 127380149 3/14/9X 3/24/9X 6390

b) Relations

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Mapping a binary 1:1 relationship

(a) Binary 1:1 relationship

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Figure (b) Resulting relations

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NULL Values in Foreign KeysNULL Values in Foreign Keys• Whether or not a Foreign Key can have Whether or not a Foreign Key can have

NULL values depends on the minimum NULL values depends on the minimum cardinality of the concerned relationshipcardinality of the concerned relationship

• Minimum cardinality of 0 represented as Minimum cardinality of 0 represented as NULL allowed for foreign key columnsNULL allowed for foreign key columns

• Minimum cardinality of 1 represented as Minimum cardinality of 1 represented as NULL disallowed for foreign key columnsNULL disallowed for foreign key columns

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Sub-tablesSub-tables• Sub-entity Sub-entity - a - a subsetsubset of another entity, called of another entity, called

the super-entitythe super-entity– Has 1-1 relationship to the super-entity (IS-A)Has 1-1 relationship to the super-entity (IS-A)– Are existence-dependent on the super-entityAre existence-dependent on the super-entity

EMPLOYEE

AGENT

CLERK

PROGRAMMER

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• Sub-entities become sub-tables in baseline logical design, but may merge with super-table in final design

• Any kind of table may have sub-table

Sub-tablesSub-tables

EMPLOYEE

PROGRAMMER

SYSTEM

ASSIGNMENT

PERMANENT

TEMPORARY

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Design of sub-tableDesign of sub-table• Foreign key identifies super-tableForeign key identifies super-table

• Primary key-foreign key with no qualifying columnsPrimary key-foreign key with no qualifying columns• Foreign key rules reflect existenceForeign key rules reflect existence

– Nulls not allowedNulls not allowed– Delete cascadesDelete cascades– Update cascadesUpdate cascades

EMPLOYEE

PROGRAMMER

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Design of sub-tableDesign of sub-tableCREATE TABLECREATE TABLE PROGRAMMER (EMP#, LANGUAGE, LEVEL)PROGRAMMER (EMP#, LANGUAGE, LEVEL)PRIMARY KEYPRIMARY KEY (EMP#)(EMP#)FOREIGN KEYFOREIGN KEY (EMP# IDENTIFIES EMPLOYEE NULLS NOT (EMP# IDENTIFIES EMPLOYEE NULLS NOT

ALLOWED DELETE OF EMPLOYEE ALLOWED DELETE OF EMPLOYEE RESTRICTED RESTRICTED UPDATE OF EMPLOYEE CASCADES)UPDATE OF EMPLOYEE CASCADES)

• Optional class column in super-table improves Optional class column in super-table improves performance, but is redundant to data in sub-tablesperformance, but is redundant to data in sub-tables

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Representing IS-A RelationshipsRepresenting IS-A Relationships

PROPERTY

MOUNTAIN PROPERTY

BEACH PROPERTY

CITY STATE ZIP

STREET ADDRESS

TYPICAL RENT

NO. ROOMS

STREET ADDRESS

STREET ADDRESS CITY

STATE ZIPCITY

STATE ZIP

SKIING

BLOCKS TO BEACH

(a) E-R diagram

IS-A IS-A

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Representing IS-A RelationshipsRepresenting IS-A Relationships

STREET ADDRESS CITY STATE ZIP NO. ROOMS TYPICAL UNIT120 Surf Dr. Honolulu, HI 99987 3 500

100 Mogul Dr. Jackson, WY 89204 3 250

PROPERTY

STREET ADDRESS CITY STATE ZIP BLOCKS TO BEACH120 Surf Dr. Honolulu, HI 99987 2

BEACH

STREET ADDRESS CITY STATE ZIP SKIING100 Mogul Dr. Jackson, WY 89204 N

MOUNTAIN

(b) Relations

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Associative TablesAssociative Tables

• M-N relationshipM-N relationship becomes associative table becomes associative table with two foreign keyswith two foreign keys

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Associative TablesAssociative Tables

• Associative tables also known as association or Associative tables also known as association or intersection tableintersection table

EMPLOYEE PROJECT

EMP# E-NAME P-NAMEP#

1 NEMPLOYEE PROJECT

EMP# E-NAME P-NAMEP#

ASSIGNMENT

EMP# P#

M N

N 1

EMP# E-NAME EMP# P# P# P-NAME

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Transforming RelationshipsTransforming Relationships

Mapping Unary RelationshipsMapping Unary Relationships What do you do when the relationship is unary?What do you do when the relationship is unary? Rules are the same irrespective of whether the Rules are the same irrespective of whether the

relationship is unary or binary. 1:N and 1:1 relationship is unary or binary. 1:N and 1:1 relationships be come foreign keys in the relationships be come foreign keys in the samesame table. M:N relationship becomes a separate table. M:N relationship becomes a separate (associative table).(associative table).

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In-Class Exercise: Transform the In-Class Exercise: Transform the following ERD to a relational structurefollowing ERD to a relational structure

EMPLOYEE

DIVISION

DEPARTMENT

SSN

EMP#

FNAME LNAME SALARY

JOBCODE

MARRIED-TO

BLDGDIVNAME

DIVNAME

DEPT#

DEPTNAME

DIRECT

WORK-IN MANAGE

BELONG-TO

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Reverse EngineeringReverse Engineering

Modify the previous ERD to Modify the previous ERD to reflect the existence of the reflect the existence of the following five tablesfollowing five tables

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More Example TablesMore Example Tables

EMP LANGUAGE LEVELE1 PL/1 BEGE6 FORTRAN INTE7 COBOL ADVE8 FORTRAN BEG

PROGRAMMER

EMP TERRITORY COMMISSIONE2 TEXAS 33000E3 NULL NULL

E11 FLORIDA 23050E12 TEXAS 1905

AGENT

PROJ# DESCRIPTION BUDGETP1 XYZ DATABASE 19000P2 GIZMO TEST 48000P3 WIDGET SALES 4000

PROJECT

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More Example TablesMore Example Tables

EMP# PROJ# HOURSE1 P1 80E1 P3 230E3 P2 NULLE5 P3 1200

ASSIGNMENT

PROJ# TASKNAME START STATUSP1 REQUIREMENTS 10/29/85 COMPLETEP1 LOGICAL DESIGN 3/24/87 ACTIVEP1 PHYSICAL DESIGN 3/29/87 PROPOSEP3 SALES REP NULL PROPOSE

TASK