Represent entities and relations with diagrams

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1 LEARNING OBJECTIVES Define data modeling terms Describe E-R Model Identify entities and relations Represent entities and relations with diagrams

2 WHAT IS DATA MODELING? A data model is a collection of concepts for describing data.

3 DATABASES MODEL THE REAL WORLD Data Model allows us to translate real world things into structures computers can store. Many models exist: Relational Object-Oriented etc.

6 STEPS IN DATABASE DESIGN Requirements Analysis user needs; what must database do? Conceptual Design high level modeling (often done w/er diagrams) Logical Design translate ER into relational data model Schema Refinement consistency, normalization

7 STEPS IN DATABASE DESIGN Physical refinement indexes Security Design who accesses what, and how

8 THE ENTITY RELATIONAL (E-R) DATA MODEL A way to make a conceptual model The E-R model assumes that any reality can be described by 1. Entities 2. The description and properties of entities (Attributes) 3. The association between the entities (Relationships)

9 THE ENTITY RELATIONAL (E-R) DATA MODEL Cont d Name Age Owns Is Owned by Make Model height Person Laptop Size

10 Entities: 1. Entity instance person, place, object, event, concept (often corresponds to a row in a table) 2. Entity Type collection of entities (often corresponds to a table) Relationships: 1. Relationship instance link between entity instances(corresponds to primary key-foreign key equivalencies in related tables) 2. Relationship type link between entity types Attributes: 1. Properties or characteristics of an entity or relationship type (often corresponds to a field in a table) 10

11 SAMPLE E-R DIAGRAM

12 EXAMPLE

13 DEFINITIONS Entity-Relationship model (E-R model): Entity-Relationship Diagram (ERD): A logical representation of the data for an organization or for a business area, using entities for categories of data and relationships for associations between entities A graphical representation of entityrelationship model

14 ENTITIES Entity is the unit of things in your business that you want to describe (store its data in the database) Defining entities is the first step to develop a conceptual model Entity is a person, place or thing for which data can be stored.

15 ENTITY TYPE Entity Type is the category that has all the entities that share similar properties (or attributes)

16 ENTITY INSTANCE Entity instance is a single occurrence of entity type, and it is the single unit that we want to store data for Entity instance must have unique value that identifies it from all other instances (e.g. serial number in case of a car).

EXAMPLE Entity Instance Property (or Attribute) Student Number Student Name Course 213434 John DBAS2323

17 EXAMPLE Entity Instance Property (or Attribute) Student Number Student Name Course John DBAS Kelly PRG Zac DBAS John PRG Steve DBAS3333 Value Entity Type is Student

DEFINITIONS Entity: A person, a place, an object, an event, or a concept in the user environment about which the organization wishes to maintain data

18 DEFINITIONS Entity: A person, a place, an object, an event, or a concept in the user environment about which the organization wishes to maintain data (Hoffer A 2013 ) Entity Type: Entity Instance: A collection of entities that share common properties or characteristics A single occurrence of an entity type

19 GUIDELINES TO RECOGNIZING ENTITIES A true data entity will have many possible instances Each of the instances will have something different that will differentiate it from other instances

Entities is where the entity instances depend on instances from other

20 STRONG VERSUS WEAK ENTITIES Strong Entity is an entity where all its instances exist independently of instances from other entity types Weak Entities is where the entity instances depend on instances from other entity type. Employee is a strong entity type Employee card is a weak entity type

21 ENTITY TYPES NOTATION IN ERD Strong Entity : Rectangle with an entity type name inside it Weak Entity : Double line rectangle with an entity type name inside it <Entity Type Name> <Entity Type Name>

22 ATTRIBUTES Is a characteristic that describes an entity Each entity instance within an entity type will have the same attributes, which are the attributes of it entity type The attribute is a holder to which we associate values (data)

23 ATTRIBUTES NOTATION IN ERD <Entity Type Name> Add the attributes inside their entity type rectangle Attribute 1 Attribute2 Attribute 3

24 SIMPLE (ATOMIC) VS COMPOSITE ATTRIBUTES Simple Attribute Difference Can not be broken to smaller meaningful components Composite Attribute Can be broken to smaller meaningful components example Age Address, Full Name

25 SINGLE VALUE VS MULTIVALUED ATTRIBUTES Single-Valued Difference Has only one value (data) associated to the attribute Multivalued Has more than one value (data) associated to the attribute example Address Employee skills

26 STORED VS DERIVED ATTRIBUTES Stored Attribute Difference Is the origin of the data. Can not recreate its associated data Derived Can be recreated from other attribute values example Date Employed Years Employed

27 IDENTIFIER ATTRIBUTE Is an attribute or a combination of more that one attributes whose value will uniquely identify each entity instance within an entity type Each entity must have an identifier Student Id, Social Insurance Number are examples of identifier attributes

28 DIFFERENT ATTRIBUTE TYPES NOTATION IN ERD EMPLOYEE

DEFINITIONS Attribute: A property or Characteristic of an entity or relationship type that is of interest to the organization Required Attribute: An attribute that must

29 DEFINITIONS Attribute: A property or Characteristic of an entity or relationship type that is of interest to the organization Required Attribute: An attribute that must have a value for every entity instance with which it is associated Optional Attribute: An attribute that may not have a value for every entity instance with which it associates

30 DEFINITIONS Composite Attribute: Simple (or Atomic) Attribute: An attribute that has meaningful component parts (attributes) An attribute that cannot be broken down into smaller components Multivalued Attribute: An attribute that may take on more than one value for a given entity (or relationship) instance

31 DEFINITIONS Derived Attribute: Identifier: An attribute whose values can be calculated from related attribute values An attribute (or a combination of attributes) whose value distinguishes instances of an entity type

32 RELATIONSHIPS Represent the interaction between entities and is commonly called Relationship Relationships are representations of the business rules in the organization For example the relation between Teacher and Student in the classroom is Teaches

33 RELATIONSHIPS TYPE AND INSTANCE Relationship type is the general definition of relationship between entity types. In other words defines what is the possible interaction between the instances of those two entity types Relationship Instance is the actual relation between the entity instances of two entity types. And there is always only one relationship instance between two entity instances

34 DEFINITIONS Relationship type: Relationship Instance: A meaningful association between (or among) entity types An association between(or among) entity instances where each relationship instance associate exactly one entity instance from each participating entity type

35 RELATIONSHIP INTSANCE

36 DEGREE OF A RELATIONSHIP Number of entity types that participate in a relationship Unary (degree 1), Binary (degree 2), and Ternary (degree 3) are the most common degrees

37 Manages DEGREE OF A RELATIONSHIP Unary Relationship EMPLOYEE Also called as recursive relationship, and it is a relation between entity instances of the same entity type

38 DEGREE OF A RELATIONSHIP Binary Relationship EMPLOYEE EQUIPMENT Relationship between instances of two entity types

39 DEGREE OF A RELATIONSHIP Ternary Relationship Relationship among the instances of three entity types. It is something that happens where the instance of three entity types join together STUDENT Meet ROOM TEACHER

40 NAMING RELATIONSHIPS Is a verb phrase Avoid vague names such as Has or Is Related

41 RELATIONSHIPS REPRESENTATION IN ERD Single line between strong entity types that it associates Double line between strong and Weak entity types that it associates relation name typed on top of the line TEACHER Teaches STUDENT Possesses STUDENT T CARD

42 DEFINITIONS Unary relationship: A relationship between instances of a single entity type Binary relationship: A relationship between the instances of two entity types Ternary relationship: A relationship among the instances of three entity types

43 CARDINALITY CONSTRAINTS Specifies the number of instances of an entity type that may or must be associated to a single instance from another entity type Cardinality is defined by minimum number of instances that may or must relate (Minimum Cardinality) and maximum instances that that may or must relate (Maximum Cardinality)

44 CARDINALITY CONSTRAINT NOTATION IN ERD Only One One or Many

45 Examples of cardinality constraints (cont.) a) One optional, one mandatory A project must be assigned to at least one employee, and may be assigned to many An employee can be assigned to any number of projects, or may not be assigned to any at all

46 Examples of cardinality constraints b) Mandatory cardinalities A patient history is recorded for one and only one patient A patient must have recorded at least one history, and can have many

47 DEFINITIONS Cardinality constraint: Minimum Cardinality: Maximum Cardinality: A rule that specifies the number of instances of one entity that can (or must) be associated with each instance of another entity The minimum number of instances of one entity that may be associated with each instance of another entity The maximum number of instances of one entity that may be associated with each instance of another entity

48 ATTRIBUTES ON RELATIONSHIPS Attributes may be associated to relationships

represented as an entity type called Associative Entity

49 ATTRIBUTES ON RELATIONSHIPS The existence of attribute on relationship means that the relation should be represented as an entity type called Associative Entity Type Associative entity is represented as curved edged rectangle

50 ATTRIBUTES FOR RELATED ENTITIES Multivalued and Composite attributes can be converted to entity types.

51 Requires MULTIVALUED ATTRIBUTES OR RELATED ENTITIES hhhh Attribute Relationship and Entity COURSE Course ID Course Title {Prerequisite} COURSE Course ID Course Title

52 COMPOSITE, MULTIVALUED ATTRIBUTES FOR RELATED ENTITIES Attribute Relationship and Entity EMPLOYEE Employee ID Employee Name EMPLOYEE Employee ID Possesses SKILL Skill Code {Skill (Skill Code, Skill Title, Skill Type)} Employee Name Skill title Skill Type

53 COMPOSITE ATTRIBUTES FOR RELATED ENTITIES Attribute Relationship and Entity EMPLOYEE Employee ID Employee Name EMPLOYEE Employee ID Works for DEPARTMENT Dept Number Dept(Dept Number, Dept Name, Dept Tel #) Employee Name Dept Name Dept Tel

54 SAMPLE PROBLEM A vendor builds multiple types of tablet computers. Each type has a type identification number and a name. The key specifications for each type include amount of storage space and display type. The company uses multiple processor types, exactly one of which is used for a specific tablet computer type; obviously, the same processor can be used in multiple types of tablets. Each processor has a manufacturer and a manufacturer s unique code that identifies it.

56 DRAW ER DIAGRAM BASED ON FOLLOWING STATEMENT A book is identified by its ISBN number, and it has a title, price, and a date of publication. It is published by a publisher, which has its own ID number and a name. Each book has exactly one publisher, but one publisher may typically publish multiple books over time.

58 DRAW ER DIAGRAM BASED ON FOLLOWING STATEMENT A book is written by one or multiple authors. Each author is identified by an author number and has a name and date of birth. Each author has either one or multiple books; in addition, occasionally data are needed regarding prospective authors who have not yet published any books.

The Entity-Relationship Model. The Entity-Relationship model. The ER model. The Entity-Relationship model. E-R Model Constructs. E-R Model Constructs

The Entity-Relationship Model. The Entity-Relationship model. The ER model. The Entity-Relationship model. E-R Model Constructs. E-R Model Constructs The Entity-Relationship Model Conceptual Data Modeling The Entity-Relationship model The E-R model is a detailed, logical representation of the data for an organisation or business area It should be understandable