8/10/2019 Entity Relations Model
http://slidepdf.com/reader/full/entity-relations-model 1/52
Dr. Kalpakis
CMSC 461, Database Management Systems
http://www.csee.umbc.edu/~kalpakis/Courses/461
Entity-Relationship odel
8/10/2019 Entity Relations Model
http://slidepdf.com/reader/full/entity-relations-model 2/52
CMSC 461- Dr. Kalpakis
2
!utline
Entity Sets
Relationship Sets
Design Issues
Mapping Constraints
Keys
E-R Diagram
Extene E-R !eatures
Design o" an E-R Database S#hema
Reu#tion o" an E-R S#hema to $ables
8/10/2019 Entity Relations Model
http://slidepdf.com/reader/full/entity-relations-model 3/52
CMSC 461- Dr. Kalpakis
3
Entity "ets
% database #an be moele as a #olle#tion o" entities, an
relationships among those entities&
%n entity is an ob'e#t that exists an is istinguishable "rom
other ob'e#ts&
Entities ha(e attributes
%n entity set is a set o" entities o" the same type that share the
same properties&
8/10/2019 Entity Relations Model
http://slidepdf.com/reader/full/entity-relations-model 4/52
CMSC 461- Dr. Kalpakis
4
#ttributes
%n entity is represente by a set o" attributes, i&e& es#ripti(e
properties possesse by all members o" an entity set&
$omain ) the set o" permitte (alues "or ea#h attribute%ttribute types*
"imple an composite attributes +that #an be "urther i(ie in
#omponent attributes&
"in%le-&alued an multi-&alued attributes $eri&ed attributes
Can be #ompute "rom other attributes
E&g& a%e, gi(en ate o" birth
customer = (customer-id, customer-name, customer-street, customer-city)
loan = (loan-number, amount)
8/10/2019 Entity Relations Model
http://slidepdf.com/reader/full/entity-relations-model 5/52
CMSC 461- Dr. Kalpakis
5
Composite #ttributes
8/10/2019 Entity Relations Model
http://slidepdf.com/reader/full/entity-relations-model 6/52
CMSC 461- Dr. Kalpakis
6
Relationship "ets
% relationship is an asso#iation among se(eral entities
% relationship set is a mathemati#al relation among n ≥
entities, ea#h ta.en "rom entity sets
/+e1, e-, 0 en e1 ∈ E 1, e- ∈ E -, 0, e
n ∈ E
n2
3here +e1, e-, 0, en is a relationship
a relationship set #an ha(e asso#iate attributeseg, the depositor relationship set bet3een entity sets customer anaccount may ha(e the attribute access-date
8/10/2019 Entity Relations Model
http://slidepdf.com/reader/full/entity-relations-model 7/52
CMSC 461- Dr. Kalpakis
7
$e%ree o' a Relationship "et
Degreethe number o" entity sets that parti#ipate in arelationship set&
Relationship sets that in(ol(e t3o entity sets are binary +or
egree t3o&
5enerally, most relationship sets in a atabase system are
binary, e(en though there are #ases 3here higher egree
relationship sets o##ur
8/10/2019 Entity Relations Model
http://slidepdf.com/reader/full/entity-relations-model 8/52
CMSC 461- Dr. Kalpakis
8
appin% Cardinalities
Express the number o" entities to 3hi#h another entity
#an be asso#iate (ia a relationship set&
Most use"ul in es#ribing binary relationship sets&
!or a binary relationship set the mapping #arinality
must be one o" the "ollo3ing types*
ne to one
ne to many
Many to one
Many to many
8/10/2019 Entity Relations Model
http://slidepdf.com/reader/full/entity-relations-model 9/52
CMSC 461- Dr. Kalpakis
9
appin% Cardinalities
One to one One to many
8/10/2019 Entity Relations Model
http://slidepdf.com/reader/full/entity-relations-model 10/52
CMSC 461- Dr. Kalpakis
10
appin% Cardinalities
Many to one Many to many
8/10/2019 Entity Relations Model
http://slidepdf.com/reader/full/entity-relations-model 11/52
CMSC 461- Dr. Kalpakis
11
E-R $ia%rams
Rectangles represent entity sets
Diamonds represent relationship sets&
Lines lin. attributes to entity sets an entity sets to relationship
sets&
Ellipses represent attributesDouble ellipses represent multi(alue attributes
Dashed ellipses enote eri(e attributes&
Underline ini#ates primary .ey attributes +3ill stuy later
7e express #arinality #onstraints by ra3ing either a ire#te
line +→, signi"ying 8one,9 or an unire#te line +:, signi"ying
8many,9 bet3een the relationship set an the entity set&
8/10/2019 Entity Relations Model
http://slidepdf.com/reader/full/entity-relations-model 12/52
CMSC 461- Dr. Kalpakis
12
E-R $ia%ram E(ample
8/10/2019 Entity Relations Model
http://slidepdf.com/reader/full/entity-relations-model 13/52
CMSC 461- Dr. Kalpakis
13
E-R $ia%ram E(ample
8/10/2019 Entity Relations Model
http://slidepdf.com/reader/full/entity-relations-model 14/52
CMSC 461- Dr. Kalpakis
14
E-R $ia%ram E(ample
8/10/2019 Entity Relations Model
http://slidepdf.com/reader/full/entity-relations-model 15/52
CMSC 461- Dr. Kalpakis
15
Roles
Entity sets o" a relationship nee not be istin#t
;se roles to spe#i"y the purpose o" ea#h entity in the relationship
Roles are ini#ate in E-R iagrams by labeling the lines that #onne#t
iamons to re#tangles&
Role labels are optional, an are use to #lari"y semanti#s o" the relationship
8/10/2019 Entity Relations Model
http://slidepdf.com/reader/full/entity-relations-model 16/52
CMSC 461- Dr. Kalpakis
16
)articipation o' an Entity "et in a Relationship "et
$otal parti#ipation +ini#ate by ouble line
e(ery entity in the entity set parti#ipates in at least one relationship in the
relationship set
<artial parti#ipation* some entities may not parti#ipate in any
relationship in the relationship set
8/10/2019 Entity Relations Model
http://slidepdf.com/reader/full/entity-relations-model 17/52
CMSC 461- Dr. Kalpakis
17
*eys
% super key o" an entity set is a set o" one or more attributes
3hose (alues uni=uely etermine ea#h entity&
% candidate key o" an entity set is a minimal super .ey
Customer-id is #aniate .ey o" customer
account-number is #aniate .ey o" account
%lthough se(eral #aniate .eys may exist, one o" the #aniate
.eys is sele#te to be the primary key
8/10/2019 Entity Relations Model
http://slidepdf.com/reader/full/entity-relations-model 18/52
CMSC 461- Dr. Kalpakis
18
*eys 'or Relationship "ets
$he #ombination o" primary .eys o" the parti#ipating entity sets
"orms a super .ey o" a relationship set&
Must #onsier the mapping #arinality o" the relationship set
3hen e#iing 3hat are the #aniate .eys
>ee to #onsier semanti#s o" relationship set in sele#ting the primary key in #ase o" more than one #aniate .ey
8/10/2019 Entity Relations Model
http://slidepdf.com/reader/full/entity-relations-model 19/52
CMSC 461- Dr. Kalpakis
19
Cardinality Constraints on +ernary Relationships
7e allo3 at most one arro3 out o" a ternary +or greater egree
relationship to ini#ate a #arinality #onstraint
More than one arro3s lea to ambiguities in the semanti#s o"
the #arinality #onstraint, an thus are not permitte
8/10/2019 Entity Relations Model
http://slidepdf.com/reader/full/entity-relations-model 20/52
CMSC 461- Dr. Kalpakis
20
,inary &s. non-,inary Relationships
Some relationships that appear to be non-binary may be better
represente using binary relationships
E&g& % ternary relationship parents, relating a #hil to his?her "ather an
mother, is best repla#e by t3o binary relationships, 'ather an mother
;sing t3o binary relationships allo3s partial in"ormation +e&g& only mother
being .no3
@ut there are some relationships that are naturally non-binary
E&g& works-on
8/10/2019 Entity Relations Model
http://slidepdf.com/reader/full/entity-relations-model 21/52
CMSC 461- Dr. Kalpakis
21
Con&ertin% on-,inary Relationships to ,inary Relationships
In general, any non-binary relationship #an be represente using
binary relationships by #reating an arti"i#ial entity set&
8/10/2019 Entity Relations Model
http://slidepdf.com/reader/full/entity-relations-model 22/52
CMSC 461- Dr. Kalpakis
22
Con&ertin% on-,inary Relationships
%lso nee to translate #onstraints
$ranslating all #onstraints may not be possible
$here may be instan#es in the translate s#hema that
#annot #orrespon to any instan#e o" R
E(ercise: add constraints to the relationships R # R , and RC to ensure that a
ne3ly #reate entity #orrespons to exa#tly one entity in ea#h o" entity sets
# , an C
7e #an a(oi #reating an ienti"ying attribute by ma.ing E a 3ea. entity
set +es#ribe shortly ienti"ie by the three relationship sets
8/10/2019 Entity Relations Model
http://slidepdf.com/reader/full/entity-relations-model 23/52
CMSC 461- Dr. Kalpakis
23
E-R $esi%n ssues
;se o" entity sets (s& attributes
Choi#e mainly epens on the stru#ture o" the enterprise being moele, an
on the semanti#s asso#iate 3ith the attribute in =uestion&
;se o" entity sets (s& relationship sets
<ossible guieline is to esignate a relationship set to es#ribe an a#tion that
o##urs bet3een entities
@inary (ersus n-ary relationship sets
%lthough it is possible to repla#e any nonbinary +n-ary, "or n A relationship
set by a number o" istin#t binary relationship sets, a n-ary relationship set
sho3s more #learly that se(eral entities parti#ipate in a single relationship&
<la#ement o" relationship attributes
8/10/2019 Entity Relations Model
http://slidepdf.com/reader/full/entity-relations-model 24/52
CMSC 461- Dr. Kalpakis
24
0eak Entity "ets
%n entity set that oes not ha(e a primary .ey is re"erre to as a weak entity
set &
$he existen#e o" a 3ea. entity set epens on the existen#e o" a identi'yin%
entity set
it must relate to the ienti"ying entity set (ia a total, one-to-many relationship
set "rom the ienti"ying to the 3ea. entity setIenti"ying relationship epi#te using a ouble iamon
$he discriminator or partial key2 o" a 3ea. entity set is the set o" attributes
that istinguishes among all the entities o" a 3ea. entity set&
$he primary .ey o" a 3ea. entity set is "orme by the primary .ey o" thestrong entity set on 3hi#h the 3ea. entity set is existen#e epenent, plus
the 3ea. entity setBs is#riminator&
8/10/2019 Entity Relations Model
http://slidepdf.com/reader/full/entity-relations-model 25/52
CMSC 461- Dr. Kalpakis
25
0eak Entity "ets
7e epi#t a 3ea. entity set by ouble re#tangles&
7e unerline the is#riminator o" a 3ea. entity set 3ith a
ashe line&
7hat happens i" payment ha attribute loan-number
8/10/2019 Entity Relations Model
http://slidepdf.com/reader/full/entity-relations-model 26/52
CMSC 461- Dr. Kalpakis
26
"peciali3ation
$op-o3n esign pro#ess 3e esignate subgroupings 3ithin an entity
set that are istin#ti(e "rom other entities in the set&
$hese subgroupings be#ome lo3er-le(el entity sets that ha(e attributes or
parti#ipate in relationships that o not apply to the higher-le(el entity set&
Depi#te by a trian%le #omponent labele IS%
$he IS% relationship is also re"erre to as superclass – subclass
relationship
Attribute inheritance
a lo3er-le(el entity set inherits all the attributes an relationship
parti#ipation o" the higher-le(el entity set to 3hi#h it is lin.e&
Can ha(e multiple spe#ialiations o" an entity set base on i""erent
"eatures&
8/10/2019 Entity Relations Model
http://slidepdf.com/reader/full/entity-relations-model 27/52
CMSC 461- Dr. Kalpakis
27
"peciali3ation E(ample
8/10/2019 Entity Relations Model
http://slidepdf.com/reader/full/entity-relations-model 28/52
CMSC 461- Dr. Kalpakis
28
enerali3ation
% bottom-up esign pro#ess
#ombine a number o" entity sets that share the same "eatures into a
higher-le(el entity set&
Spe#ialiation an generaliation are simple in(ersions o" ea#h
other they are represente in an E-R iagram in the same 3ay&
$he terms spe#ialiation an generaliation are use
inter#hangeably&
8/10/2019 Entity Relations Model
http://slidepdf.com/reader/full/entity-relations-model 29/52
CMSC 461- Dr. Kalpakis
29
"# $esi%n Constraints
Constraint on 3hi#h entities #an be members o" a gi(en lo3er-
le(el entity set&
#onition-e"ine
user-e"ine
Constraint on 3hether or not entities may belong to more thanone lo3er-le(el entity set 3ithin a single generaliation&
Dis'oint
an entity #an belong to only one lo3er-le(el entity set
note in E-R iagram by 3riting dis5oint next to the IS% triangle
(erlapping
an entity #an belong to more than one lo3er-le(el entity set
8/10/2019 Entity Relations Model
http://slidepdf.com/reader/full/entity-relations-model 30/52
CMSC 461- Dr. Kalpakis
30
"# $esi%n Constraints
Completeness #onstraint
spe#i"ies 3hether or not an entity in the higher-le(el entity set must
belong to at least one o" the lo3er-le(el entity sets 3ithin a
generaliation&
total * an entity must belong to one o" the lo3er-le(el entity sets
partial* an entity nee not belong to any o" the lo3er-le(el entity sets
8/10/2019 Entity Relations Model
http://slidepdf.com/reader/full/entity-relations-model 31/52
CMSC 461- Dr. Kalpakis
31
#%%re%ation
Suppose 3e 3ant to re#or managers "or tas.s per"orme by an employee at
a bran#h
8/10/2019 Entity Relations Model
http://slidepdf.com/reader/full/entity-relations-model 32/52
CMSC 461- Dr. Kalpakis
32
#%%re%ation
Relationship sets works-on an mana%es represent o(erlapping in"ormation
E(ery mana%es relationship #orrespons to a works-on relationship
Fo3e(er, some works-on relationships may not #orrespon to any mana%es
relationships
So 3e #anBt is#ar the works-on relationship
Eliminate this reunan#y (ia a%%re%ation$reat relationship as an abstra#t entity
%llo3s relationships bet3een relationships
%bstra#tion o" relationship into ne3 entity
7ithout introu#ing reunan#y, the "ollo3ing iagram represents*%n employee 3or.s on a parti#ular 'ob at a parti#ular bran#h
%n employee, bran#h, 'ob #ombination may ha(e an asso#iate manager
8/10/2019 Entity Relations Model
http://slidepdf.com/reader/full/entity-relations-model 33/52
CMSC 461- Dr. Kalpakis
33
E-R $ia%ram 0ith #%%re%ation
8/10/2019 Entity Relations Model
http://slidepdf.com/reader/full/entity-relations-model 34/52
CMSC 461- Dr. Kalpakis
34
$esi%n )hases
Spe#i"i#ation o" user re=uirements
5ather userBs ata re=uirements
5ather userBs "un#tional re=uirements
Con#eptual esign
$ranslate the userBs ata re=uirement into a #on#eptual s#hema "or the #hosen
ata moel to #reate a etaile o(er(ie3 o" the userBs enterpriseRe(ie3 "or in#onsisten#ies an #o(erage
Gogi#al esign
Map the #on#eptual s#hema onto the ata moel o" the #hosen atabase system
<hysi#al esignSpe#i"y the physi#al #hara#teristi#s o" the implementation o" the logi#al s#hema
8/10/2019 Entity Relations Model
http://slidepdf.com/reader/full/entity-relations-model 35/52
CMSC 461- Dr. Kalpakis
35
E-R $esi%n $ecisions
$he use o" an attribute or entity set to represent an ob'e#t&
7hether a real-3orl #on#ept is best expresse by an entity set
or a relationship set&
$he use o" a ternary relationship (ersus a pair o" binary
relationships&
$he use o" a strong or 3ea. entity set&
$he use o" IS% ) #ontributes to moularity in the esign&
$he use o" aggregation ) #an treat the aggregate entity set as asingle unit 3ithout #on#ern "or the etails o" its internal
stru#ture&
8/10/2019 Entity Relations Model
http://slidepdf.com/reader/full/entity-relations-model 36/52
CMSC 461- Dr. Kalpakis
36
,ankin% Enterprise - $ata Reuirements
@an.
is organie into bran#hes
ea#h bran#h is lo#ate in a parti#ular #ity an has a uni=ue name
Customers
are ienti"ie by an i
Fa(e a##ounts an?or loans
May be asso#iate 3ith a parti#ular ban. employee as loan o""i#er?personal
ban.er
@an. stores #ustomerBs name an aress
EmployeesIenti"ie by an i
Store his?her name, telephone numbers, names o" epenents, start ate,
manager, an length o" employment
8/10/2019 Entity Relations Model
http://slidepdf.com/reader/full/entity-relations-model 37/52
CMSC 461- Dr. Kalpakis
37
,ankin% Enterprise - $ata Reuirements
Accounts
%re ienti"ie by an i
Can be #he#.ing or sa(ings
May be hel by multiple #ustomers
@an. maintains a##ountBs balan#e an ate o" last a##ess
Ea#h sa(ings a##ount has an interest rate
@an. maintains the o(erra"t amount "or ea#h #he#.ingLoans
%re ienti"ie by an i an originate at a parti#ular bran#h
May be hel by multiple #ustomers
Maintain loan amount an payments
!or ea#h payment "or a loan, the ban. maintains the ate an amount o" payment
Ea#h loan payment is asso#iate 3ith a parti#ular loan an is ienti"ie by a
payment number, uni=ue among the payments "or that loan
8/10/2019 Entity Relations Model
http://slidepdf.com/reader/full/entity-relations-model 38/52
CMSC 461- Dr. Kalpakis
38
E-R $ia%ram 'or a ,ankin% Enterprise
8/10/2019 Entity Relations Model
http://slidepdf.com/reader/full/entity-relations-model 39/52
CMSC 461- Dr. Kalpakis
39
"ummary o' E-R "ymbols
8/10/2019 Entity Relations Model
http://slidepdf.com/reader/full/entity-relations-model 40/52
CMSC 461- Dr. Kalpakis
40
"ummary o' E-R "ymbols
8/10/2019 Entity Relations Model
http://slidepdf.com/reader/full/entity-relations-model 41/52
CMSC 461- Dr. Kalpakis
41
#lternati&e E-R otations
8/10/2019 Entity Relations Model
http://slidepdf.com/reader/full/entity-relations-model 42/52
CMSC 461- Dr. Kalpakis
42
Reduction o' an E-R "chema to +ables
<rimary .eys allo3 entity sets an relationship sets to be
expresse uni"ormly as tables 3hi#h represent the #ontents o"the atabase&
% atabase 3hi#h #on"orms to an E-R iagram #an be represente by a
#olle#tion o" tables&
!or ea#h entity set an relationship set there is a uni=ue table
3hi#h is assigne the name o" the #orresponing entity set or
relationship set&
Ea#h table has a number o" #olumns +generally #orresponing to
attributes, 3hi#h ha(e uni=ue names&
Con(erting an E-R iagram to a table "ormat is the basis "or
eri(ing a relational atabase esign "rom an E-R iagram&
8/10/2019 Entity Relations Model
http://slidepdf.com/reader/full/entity-relations-model 43/52
CMSC 461- Dr. Kalpakis
43
Representin% Entity "ets as +ables
% strong entity set reu#es to a table 3ith the same attributes
8/10/2019 Entity Relations Model
http://slidepdf.com/reader/full/entity-relations-model 44/52
CMSC 461- Dr. Kalpakis
44
Composite #ttributes
Composite attributes are "lattene out by #reating a separate
attribute "or ea#h #omponent attribute
E&g& gi(en entity set customer 3ith #omposite attribute name 3ith
#omponent attributes 'irst-name an last-name the table #orresponing
to the entity set has t3o attributes
name.'irst-name an name.last-name
8/10/2019 Entity Relations Model
http://slidepdf.com/reader/full/entity-relations-model 45/52
CMSC 461- Dr. Kalpakis
45
ulti-&alued #ttributes
% multi-(alue attribute M o" an entity E is represente by a
separate table EM
$able EM has attributes #orresponing to the primary .ey o" E an an
attribute #orresponing to multi-(alue attribute M
e&g& Multi-(alue attribute dependent-names o" employee is represente by
a table
employee-dependent-names+employee-id dname
Ea#h (alue o" the multi-(alue attribute maps to a separate ro3 o" the
table EM
E&g&, an employee entity 3ith primary .ey Hohn anepenents Mar. an Mary maps to t3o ro3s*
+Hohn, Mar. an +Hohn, Mary
8/10/2019 Entity Relations Model
http://slidepdf.com/reader/full/entity-relations-model 46/52
CMSC 461- Dr. Kalpakis
46
Representin% 0eak Entity "ets
A weak entity set becomes a table that includes columns for the
primary key of the identifying strong entity set
8/10/2019 Entity Relations Model
http://slidepdf.com/reader/full/entity-relations-model 47/52
CMSC 461- Dr. Kalpakis
47
Representin% Relationship "ets as +ables
% many-to-many relationship set is represente as a table 3ith
#olumns "or the primary .eys o" the t3o parti#ipating entity
sets, an any es#ripti(e attributes o" the relationship set&
E&g&* table "or relationship set borrower
8/10/2019 Entity Relations Model
http://slidepdf.com/reader/full/entity-relations-model 48/52
CMSC 461- Dr. Kalpakis
48
Redundancy o' +ables
Many-to-one an one-to-many relationship sets that are total on the many-
sie #an be represente by aing an extra attribute to the many sie,#ontaining the primary .ey o" the one sie
!or one-to-one relationship sets, either sie #an be #hosen to a#t as the8many9 sie
I" parti#ipation is partial on the many sie, repla#ing a table by an extraattribute in the relation #orresponing to the 8many9 sie #oul result in null(alues
$he table #orresponing to a relationship set lin.ing a 3ea. entity set to itsienti"ying strong entity set is reunant&
" l bl
8/10/2019 Entity Relations Model
http://slidepdf.com/reader/full/entity-relations-model 49/52
CMSC 461- Dr. Kalpakis
49
Representin% "peciali3ation as +ables
Method 1
!orm a table "or the higher le(el entity
!orm a table "or ea#h lo3er le(el entity set, that in#lues
primary .ey o" higher le(el entity set an lo#al attributes
table table attributes
person name street city
customer name credit-ratin%
employee name salary
Dra3ba#.* getting in"ormation about, e&g&, employee
re=uires a##essing t3o tables
8/10/2019 Entity Relations Model
http://slidepdf.com/reader/full/entity-relations-model 50/52
CMSC 461- Dr. Kalpakis
50
Representin% "peciali3ation as +ables
Method 2
!orm a table "or ea#h entity set 3ith all lo#al an inherite attributes
table table attributes
person name street city
customer name street city credit-ratin%
employee name street city salary
I" spe#ialiation is total, table "or generalie entity + person not
re=uire to store in"ormation
Can be e"ine as a 8(ie39 relation #ontaining union o" spe#ialiation tables@ut expli#it table may still be neee "or "oreign .ey #onstraints
Dra3ba#.* street an #ity may be store reunantly "or persons 3ho
are both #ustomers an employees
8/10/2019 Entity Relations Model
http://slidepdf.com/reader/full/entity-relations-model 51/52
CMSC 461- Dr. Kalpakis
51
Relations Correspondin% to #%%re%ation
$o represent relationships 3ith aggregate entities, #reate a
table #ontaining
$he primary .ey o" the aggregate entity
the primary .ey o" the asso#iate entity
%ny es#ripti(e attributes o" the relationship
Example
mana%es+employee-id branch-name 5ob-title mana%er-name
$able works-on is reunant if null (alues "or attribute mana%er -name intable mana%es are allo3e
8/10/2019 Entity Relations Model
http://slidepdf.com/reader/full/entity-relations-model 52/52
E-R $ia%ram 7 ,ookstore E(ample
Top Related