COMP 244. Entity Relationship Model Basics. Entity-Relationship Models. Key elements of the E-R model DATABASE CONCEPTS & APPLICATIONS

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1 COMP 244 DATABASE CONCEPTS & APPLICATIONS Entity Relationship Model Basics Key elements of the E-R model Entities Attributes Identifiers (Keys) Relationships Entity-Relationship Models 1 2 Database Concepts & Applications 1

2 Entities Entity: Any Real-world object distinguishable from other objects, about which we wish to record information. An entity may be an object with a physical existence or may be an object with conceptual existence. Entity Sets Entities of a given type are grouped into entity classes or entity sets (for example, entity class collects all entities) All entities in an entity set have the same set of attributes. Each attribute has a domain. Each entity set has a key. 3 4 Database Concepts & Applications 2

3 Strong Entity Entity Types An entity that exists independent of other entity types. Weak Entity Invoice An entity type whose existence depends on some other entity. Invoice_Items Attributes Entities have attributes (also called properties) that describe the entity s characteristics. Example: Attributes of entity : ID Name Age ID name Age 5 6 Database Concepts & Applications 3

4 Attributes Examples of Attribute Domains Attributes have domains of possible values Example: The domain of Name is the set of 20-character strings Example: The domain of Age is the set of integers from 20 to Database Concepts & Applications 4

5 Multi-Valued Attributes An attribute that may take on more than one value for a given entity instance Example: Qualifications, Skills Original E-R model allowed multi-valued attributes. Most of the implementations of E-R models don t allow multi-valued attributes. Composite Attributes An attribute that can be broken down into component parts Examples: Adress (Street, City, State, PostalCode) Name (First Name, Middle Name, Last Name) 9 10 Database Concepts & Applications 5

6 Stored and Derived Attributes Stored Attribute: An attribute whose values are stored in the database. Derived Attribute: An attribute whose values can be calculated from related attribute values. Examples: Years Employed (using EmploymentDate) Age (using DOB) Keys Entities have keys that identify their instances. The key of an entity instance consists of one or more of the entity s attributes. A key is a minimal set of attributes that uniquely identify an entity in the set. Keys can be either unique or nonunique. Examples: Number (most likely unique) will identify instances Database Concepts & Applications 6

7 Keys Entity instances can be identified by more than one candidate keys. We designate one of them as the primary key. Keys that consist of two or more attributes are called composite keys. Examples of composite keys: (AreaCode, LocalNumber) (First, Second, Third, Tribe) Relationships Entities can be associated with one another using relationships. The E-R model can contain relationship sets that associate entity sets and relationship instances that associate entity instances Database Concepts & Applications 7

8 Relationship Types Relationships can include many entities. The number of entities in a relationship is called the degree of the relationship. Relationships of degree 1 are called unary relationships. Relationships of degree 2 are called binary relationships. Relationships of degree 3 are called ternary relationships. Unary Relationships A relationship between the instances of a single entity type e.g. Person is married to a Person manages s manages Database Concepts & Applications 8

9 Binary Relationships A relationship between the instances of two entity types e.g. An works for a Department Binary Relationship Types 1:1 ONE to ONE A Department has A Manager () Works-for Department An manages A Department 1 1 manages Department Database Concepts & Applications 9

10 Binary Relationship Types 1:N ONE to MANY A Department has Many s Binary Relationship Types N:M MANY to MANY An works on Many Projects An works for A Department A Project has Many s N 1 Works-for Department Project N has M Database Concepts & Applications 10

11 Ternary Relationships An e works in a single Location l of a single Department d Ternary Relationships A Supplier s supplies Part p to Project j Supplier Supplies Part Department Works_In Location Project Database Concepts & Applications 11

12 Participation Constraints Total Participation Every must work for a Department Total Participation is identified by a thick line Participation Constraints Partial Participation Some s manage Departments Partial Participation is identified by a thin line Department Works_In Manages Department Database Concepts & Applications 12

13 Multiple Relationships In some situations, an organization may want to model more than one relationship between the same entity types teaches Instructor Student advises 25 Database Concepts & Applications 13