Database Design ProcessDatabaseRequirementsConceptual DesignConceptualModelLogical DesignPhysical DesignLogicalSchemaE-R ModelingChoice of aDBMSData ModelMapping

Database Systems

Entity-Relationship ModelAn entity is a collection of real-world objects that have many common properties.Examples: Students, Instructors, Courses, SectionsStudent entities have properties: name, address, major, graduation-yearA student may be John Smith, 22 Sage Rd., Computer Science, 2000An attribute is a data item that describes a property of an entity

Database Systems

EntitiesStudentssidstudent_namemid_initialfirstnamelastnamehobbiesprimary identifiermulti valued attributecomposite attribute

Database Systems

Mapping Entities to RelationsEach entity in an E-R model is mapped to a separate relationthe primary identifier is mapped to the primary key (underlined!)all regular attributes are mapped to an attribute in the tableeach subpart of a composite attribute is mapped to a different attributeeach multi-valued attributed is mapped to a separate relation that inherits the primary key of the parent relation

Database Systems

Mapping EntitiesStudentssidstudent_namemid_initialfirstnamelastnamehobbiesStudents( sid, lastname, firstname, mid_initial )

Hobbies( sid, hobby )Foreign key sid referencesthe Students relation!

Database Systems

Map the Auction EntitiesOwnersoidowner_namemid_initialfirstnamelastnameemailphone#itemsoldBuyersbuyidbuyer_nameemailphonecc_numaddressstreetcitystatezip

Database Systems

Map the Auction EntitiesItemsiidnamelocationdescriptionBidsbidamountdate/timedatetime

Database Systems

RelationshipsGiven a set of entities E1,E2,,Ek, a relationship R defines a rule of correspondence between these entities.An instance r(e1,e2,,ek) of the R relation means entities e1,e2,,ek are in a relation r at this instance.If two people are married, they are in a relationship:married(Bob, Margaret)If a student A takes a course C offered by professor B, then A,B,C are in a relationship.

Database Systems

RelationshipsItemsBuyersBidsplaceownOwnersacceptdatebinary relationshipternary relationship

Database Systems

Cardinalities of RelationshipsParticipation cardinalities of a relationship R for an entity E are:min-card(E, R) : the minimum number of entities in E that should be mapped via Rmax-card(E, R): the maximum number of entities in E that can be mapped via ROwn is a relation between owner and itemShould each owner be selling items?How many items can an owner sell?

Database Systems

Cardinalities of RelationshipsE R FE R FE R FOne-to-one relationshipmin-card(E, R)=0max-card(E,R)=1min-card(F,R)=0max-card(F,R)=1Many-to-one relationshipmin-card(E, R)=0max-card(E,R)=Nmin-card(F,R)=1max-card(F,R)=1Many-to-many relationshipmin-card(E, R)=0max-card(E,R)=Nmin-card(F,R)=0max-card(F,R)=N

Database Systems

CardinalitiesItemsBuyersBidsplaceownOwnersacceptdate(1,1)(0,N)(0,N)(0,1)(0,N)(1,1)(0,N)

Database Systems

CardinalitiesIf max-card(E,R)=1 then E has single-valued participation in RIf max-card(E,R)=N then E has multi-valued participation in RGiven a binary relation R between E and F, R is said to beone-to-one if both E and F have single-valued participationone-to-many if E has single and F has multi-valued participationmany-to-many if both E and F have multi-valued participation

Database Systems

Mapping Relationships to RelationsMap one-to-one and one-to-many (or many-to-one) relationships into the existing relations (derived from entities)If E-R-F is one-to-many, then include the primary key of the relation for F in the relation for E.If E-R-F is one-to-one, then include key for E in F, or the key for F in E.If E-R-F is many-to-many, create a new relation for R that has the primary keys for both E and F.If R has attributes, migrate them to the relation with the foreign keys!Contains Foreign Key(s)

Database Systems

Mapping the Auction DatabaseOwners( oid, itemsold, lastname, firstname, mid_initial, phone )

OwnerEmail( oid, email )

Buyers( buyid, buyername, ccnum,street, state, city, zip, phone )

BuyerEmail( buyid, email )

Items( iid, name, location, description, oid )

Bids( bid, date, time, amount, acceptingoid, acceptdate, buyid, iid )

Database Systems

CardinalitiesItemsBuyersBidsplaceownOwnersacceptdate(1,1)(0,N)(0,N)(0,1)(0,N)(1,1)(0,N)

Database Systems

ProblemConsider the design of a database to manage airline reservations:For flights, it contains the departure and arrival airports, dates and timesFor flights, it also contains a number of different pricing plans with different conditions (Saturday stay, advance booking, etc.)For passengers, it contains the name, telephone number and seat type preferenceReservations include the seat assigned to a passengerPassengers can have multiple reservations

Database Systems

Solutionairportflightpassengerdepartarrivedatedatetimetimereservationseatpricing plannameconditionsnamephoneseat pref(0,N)(0,N)(0,N)(0,N)(1,1)(1,1)

Database Systems

Entity Relationship ModelEntities play different roles in a relationshipEmployeessupervisesSupervised-by(1, 1)Supervisor-of(0, N)Employeessupervisessupervised(0, N)Projectsemployee-supervisor(1, 1)project-supervisor(1, N)Recursiverelationshipeid

Database Systems

Entity Relationship ModelMany-to-many relationships are translated into new relationsItemsBuyersStoresbuy(0,N)(0,N)(0,N)amount

Database Systems

BUY

Item

Buyer

Store

Amount

I1

B1

S1

3

I2

B1

S2

4

I3

B4

S1

5

I2

B5

S2

2

Entity Relationship ModelTernary relationships may be represented by binary relationshipsItemsBuyersStoresbuy_from(0,N)buy_itemsell_item(0,N)(0,N)(0,N)(0,N)(0,N)Is this conceptuallyequivalent to the previous ternaryrelationship?

Database Systems

Weak EntitiesThe existence of a weak entity W depends on the existence of another (strong) entity E through a relationship R.(Alternate) Two different weak entities may have the same identity (key) if they are related to two different strong entities.BranchBankhas(0,N)(1,1)namenumberaddress

Database Systems

Weak EntitiesWeak entities can be mapped to the relational model by:Map each weak entity E that depends on a strong entity F to a new relation RRelation R contains all the attributes in E and the primary key of FThe primary key for R is the primary key of E and the primary key of F

Database Systems

Generalization HierarchiesLower items inherit attributes of their parentsConcertsdateClassicalpiecesconductorsoloistsOtherperformerslocationorchestraOption 1. Translate intoa single relation with a flag for the type of entity[many null values]

Option 2. Translate into three entities and two is-a relationships, then translate the resulting graph.

Database Systems

ExtensionsAll relational DBMSs come with extensions that give more flexibility to the DBAExamples from Informixcomposite attributes -> translate as a record address of type ROW(street string, city string, state string, zip string) multi-valued attributes -> translate into collection types such as sets, lists, multi-sets (bags)hierarchies -> create typed tables and translate into a type hierarchy.REMEMBER, the extensions complicate the data model and make certain SQL queries much harder or impossible, leaving the database programmer with a much harder job of maintaining the database!

Database Systems

Lecture 3 - Entity Relationship Model