1. Entity-Relationship Modelling
Entity and relationship sets
Issues for object-based modelling
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CS319 Theory of Databases

2. The Entity-Relationship (E-R) Model 1
Designing a database = real-world modelling
query reflects a real-world situation
(e.g. loan status of a book)
entity-relationship model of an enterprise
reflects its overall logical structure
entity = object that exists and is distinguishable from other objects
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CS319 Theory of Databases

3. The Entity-Relationship (E-R) Model 2
… entity = object that exists and is distinguishable from other objects
cf OOP object doesn't "exist" until an instance is created
entity can be abstract or concrete
concrete person, book
abstract marriage, journey
NB abstract used here not as in OO class abstraction
6/6/2019
CS319 Theory of Databases

4. The Entity-Relationship (E-R) Model 3
Entity sets
entity set = set of entities of the same type
e.g. entity set customer
= all persons having an account at bank
entity sets can intersect
e.g. person / customer / employee
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CS319 Theory of Databases

5. The Entity-Relationship (E-R) Model 4
Entity is represented by a set of attributes
e.g. customer has c-name, ss#, street, c-city
Representation for a particular customer
{ (name, Brill),
(ss#, ),
(street, Putnam),
(city, San Marcos) }
permissible values for each attribute define its domain
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CS319 Theory of Databases

6. The Entity-Relationship (E-R) Model 5
entity set  variable entity type  type definition
Sample entity sets
branch b-name, b-city, assets
customer c-name, ss#, street, c-city
employee e-name, phone#
account account#, balance
transaction transaction#, date , amount
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CS319 Theory of Databases

7. The Entity-Relationship (E-R) Model 6
Relationships
relationship = association among several entities
e.g. a relationship associates Brill with account 183
NB refer to this association as a relationship
cf mathematical relation
{ (a,b,..., z) | aÎA, bÎB, ...., zÎZ }
relationship set = math relation on 2 or more entity sets
6/6/2019
CS319 Theory of Databases

8. The Entity-Relationship (E-R) Model 7
… relationship set = math relation on 2 or more entity sets
binary relationship if involves two entity sets
e.g. CustAcc [ = has (customer, account) ]
... relationship set contains (Brill, 183)
... can have descriptive attributes associated with a relationship: e.g. last-access-date on CustAcc
ordering in a relationship may be significant:
cf. WorksFor on employees.
 refer to role to distinguish when domain doesn't
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CS319 Theory of Databases

9. The Entity-Relationship (E-R) Model 8
How to devise E-R model? … no fixed rules:
choice of relationship sets reflects real-world semantics
Compare employee: e-name, ss#, phone#
with employee: e-name, ss#
phone: phone#, location
telrel = uses (employee, phone)
Issues
does every employee have a phone?
do employees have access to many phones?
do employees share phones?
6/6/2019
CS319 Theory of Databases

10. The Entity-Relationship (E-R) Model 9
How to devise E-R model?
What is entity / attribute set ...?
No easy answer: depends on structure of the enterprise
Cf employee: e-name, ss#, phone#
does not acknowledge employees without phones
Could associate e-name with independent entity
where employee  e-name, ss#
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CS319 Theory of Databases

11. The Entity-Relationship (E-R) Model 10
Mapping cardinalities [CustAcc = has (customer, account)]
1-1 each customer has exactly one account
no joint accounts
1-many a customer can have more than one account
many-1 every customer has exactly one account,
possibly a joint account
many-many
a customer can have more than one account,
can also have joint accounts
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CS319 Theory of Databases

12. The Entity-Relationship (E-R) Model 11
Existence dependencies
Can have an entity that exists only if another entity exists e.g. a transaction on an account
delete account
 delete transactions from log but not v.v.
existence of x depends on existence of y
 y dominant entity, x subordinate entity
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CS319 Theory of Databases

13. The Entity-Relationship (E-R) Model 12
The notion of identity
… keys for entities and relationships
"from a db perspective”
difference between individual entites & relationships must be expressed in terms of their attributes
cf observing objects in different states
that happen to coincide "from time to time"
6/6/2019
CS319 Theory of Databases

14. The Entity-Relationship (E-R) Model 13
The notion of identity ...
… keys for entities and relationships
superkey
= set of attributes that (taken together) identify an entity
e.g. ss# or {ss#,c-name} for a customer
but not c-name alone
... 2 people can have the same name, but not same social security number
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CS319 Theory of Databases

15. The Entity-Relationship (E-R) Model 14
The notion of identity
superkey without extra attributes is a candidate key
candidate key = set of attributes that
identifies an entity
and is minimal wrt this property
normally select a special key from the candidate keys
as means to identify entity: call this the primary key
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CS319 Theory of Databases

16. The Entity-Relationship (E-R) Model 15
Strong vs weak entity sets
an entity set with enough attributes to make a candidate key is a strong entity set
an entity set that doesn't have enough attributes to make a candidate key is a weak entity set
e.g. transaction#, date, amount
may coincide on two separate accounts
weak entity sets are existence dependent on strong entity sets e.g. transaction on account
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CS319 Theory of Databases

17. The Entity-Relationship (E-R) Model 16
weak entity sets are existence dependent on strong entity sets ...
i.e where there is a weak entity set
"there must be something around that can distinguish conceptually different entities"
To distinguish entities in weak entity sets:
1. identify the strong entity set on which it is existence dependent,
2. find an attribute that will discriminate between entities in the weak entity set if the strong entity is known.
For example: (account#, transaction#)  transaction.
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CS319 Theory of Databases

18. The Entity-Relationship (E-R) Model 17
Keys for relationships
Keys distinguish entities in entity sets
… what about relationships?
A key for a relationship is derived from keys for its
constituent entity sets. For instance, if R(A,B,...,Z) is a
relationship set a key for R is
“a piece of info about one or more of the entities participating in a relationship in R that is sufficient
to identify the relationship entirely "
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CS319 Theory of Databases

19. The Entity-Relationship (E-R) Model 18
Keys for relationships ….
To construct a key for a relationship
take union of the sets of attributes
primary_key(A),
primary_key(B),
... ,
primary_key(Z),
descAttr(R)
and prune via functional dependency.
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CS319 Theory of Databases

20. The Entity-Relationship (E-R) Model 19
Illustrating key construction
e.g. R=CustAcc gives
attribute(R) = {ss#, account#, date}
CustAcc many-1  candidate key is ss#
... customer has at most one account
& the date associated with this account is uniquely spec by account#
If CustAcc many-many need {ss#, account#} as key.
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CS319 Theory of Databases

21. The Entity-Relationship (E-R) Model 21
Illustrating key construction
In general:
add such attributes from descAttr(R)
as can't infer via functional dependency
e.g. CustBanker relationship has ROLE field:
loan officer / personal banker
 can't infer which role banker plays for a customer
and hence must add ROLE attribute to the key
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CS319 Theory of Databases

22. Issues for object-based modelling 1
criterion What is an entity?
can a relationship be entity?
identity how to tell the difference between entities?
effect of evolution
of entity on identity?
relationships what connections are there between entities?
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CS319 Theory of Databases

23. Issues for object-based modelling 2
relationships what connections are there between entities?
existence dynamic vs static instantiation
dependencies existence and functional
multiple views what is whole of an entity?
can we circumscribe
its set of attributes?
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CS319 Theory of Databases

24. Issues for object-based modelling 3
behaviour do entities have state and behaviour?
time do entities change, or just come and go?
hierarchy inheritance, classification
Note: entity-relationship modelling, object-oriented modelling, network and relational database design all address these issues in quite different ways.
6/6/2019
CS319 Theory of Databases