Lecture 2: Entity Relationship Data Model 2. 1

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1 Lecture 2: Entity Relationship Data Model 2. 1

2 Objectives to understand the basic constructs of the entity-relationship data model to understand the graphical symbols used by the model to represent these constructs to learn how to define the conceptual structure of the data in a database using the entity-relationship data model 2. 2

3 Contents general philosophy of the ER model entities relationships attributes keys arity of relationship types cardinality of relationship types participation constraints existential dependencies different kinds of entity types Is-a relationships 2. 3

4 General Philosophy of the ER Model everything in the real world as an entity or a relationship among two or more entities Student Does StudiesAt University Has Course Offers Department 2. 4

5 Entities entity: an existing physical or conceptual object that can be distinguished from other objects G. Burnside instance of entity type: an abstract description of the common characteristics of similar entities entity set (extension): the set of the entities, which conform to the definition of an entity type and therefore could be considered as instances of this type BankCustomer instance of instance of D. Till S. Minter 2. 5

6 Relationships relationship: an association among two or more entities relationship type: an abstract description of similar relationships among entities of the same entity types Customer (G.Burnside, Account1) instance of CustAcc Account relationship set (extension): the set of the relationships, which conform to the definition of relationship type and therefore could be considered as instances of this type 2. 6 (D.Till, Account1) (P.Smith, Account2)

7 Attributes attribute: a characteristic (property) of an entity or a relationship Customer CustAcc Account Surname Firstname Name Date Code Account attributes may take one or more primitive or composite values 2. 7

8 key (superkey): Keys (1) a minimal set of one or more attributes, which collectively identify the individual entities and relationships in the extensions of entity and relationship types Bank Code Vehicle RegNumber candidate keys: distinct collections of attributes that may serve as keys EngSerNum Vehicle ChassisSerNum 2. 8

9 primary key: Keys (2) a candidate key chosen by a database designer to be the key for an entity or relationship type EngSerNum Vehicle ChassisSerNum keys for relationship types: RegNumber a relationship type with no candidate keys may use the conjunction of the keys of entities types it associates as its own key Code Bank N BC M Customer Code 2. 9

10 Arity of Relationship Types the arity or degree of a relationship type is the number of the distinct entity types which are associated by it Customer CustAcc Account binary relationship (G.Smith, account1) Customer CustAcc Account 3-ary relationship Bank (G.Smith, Warburg, account1) 2. 10

11 Cardinality of Relationship Types the cardinality expresses the number of entities which another entity can be associated with via a relationship one-to-one (1:1): an employee has a unique security number one-to-many (1:N): bank customers have one or more credit cards but no card may be owned by more than one customer many-to-many (M:N): customers may have one or more accounts alone or jointly with other customers Employee BankCustomer 1 ESN 1 1 N Holds SecNumber CreditCard N M Bank Keeps Account 2. 11

12 Participation Constraints total-relationships: every entity must belong to at least one relationship of a total relationship type every bank account must have a code Account 1 AccCode 1 Code partial-relationship: there might be entities not belonging to any relationship in the extension of a partial relationship type there might be bank accounts having no transactions Account 1 N AccTran Transaction 2. 12

13 Existential Dependencies an existential dependency is a constraint requiring that the existence of the instances of one of the entity types in a binary relationship depends on the existence of the other entity type a RT which conveys an existential-dependency: transactions associated with bank accounts Account 1 N AccTran Transaction a RT which does not convey an existential-dependency: banks have customers Bank N HasCust M Customer 2. 13

14 Different Kinds of Entity Types strong (or dominant) entity type: the existentially independent entity type in a relationship type which conveys an existential dependency ACode Account 1 weak (or subordinate) entity type: the existentially dependent entity type in a relationship type which conveys an existential dependency discriminator: an attribute of a weak entity type, which jointly with the primary key of its dominant entity type identifies the entities in its extension AccTran N Transaction TCode 2. 14

15 Customer 1 N CA Is-a Relationships generalisation (Is-a) relationship: a binary relationship between two entity types which denotes that one of these types is more general than the other Account code balance Name Is-a Int-Rate SavingAccount CurrentAccount overdraft Is-a relationships have a subset semantics, that is the set of the instances of the subtype is a subset of the set of the instances of the supertype 2. 15

16 Is-a Relationships inheritance: Is-a relationships enforce the inheritance of attributes and relationships associated with the supertype to the subtype code Customer 1 N CA Account balance Name inherits Is-a inherits CurrentAccount overdraft distribution of attributes/relationships: the attributes/relationships which are common to both the subtype and the supertype are associated with the supertype the attributes/relationships which apply only to the subtype are associated with 2. 16

17 Main Points the ER model provides general purpose constructs useful for the conceptual design of databases: entities, relationships, attributes cardinality and participation constraints existential dependencies Is-a relationships the ER model uses special graphical symbols to denote each of these constructs these symbols should be used consistently to convey unambiguously the meaning of an ER diagram to its users 2. 17

Conceptual Data Modeling

Conceptual Data Modeling Conceptual Data odeling A data model is a way to describe the structure of the data. In models that are implemented it includes a set of operations that manipulate the data. A Data odel is a combination