IS 4420 Database Fundamentals Chapter 3: Modeling Data in the Organization Leon Chen

Systems Development Life Cycle Database Development Process Enterprise modeling Project Identification and Selection Conceptual data modeling Project Initiation and Planning Analysis Logical Design Logical database design Physical database design and definition Physical Design Implementation Database implementation Maintenance Database maintenance

Overview What is data modeling Importance of data modeling Business rules Entity-relationship model: entities, relationships, and attributes

Conceptual Data Modeling Business Rules Data names Data definitions Conceptual Data Model (Entity-Relationship Diagram)

Data Modeling is the Most Important Input for follow-up processes Data rather than processes are the most complex Data tend to be more stable than business processes

Business Rules Statements that define or constrain some aspect of the business. For example: Friday is business casual dress day Students can only register a course if prerequisite is successfully completed Automated through DBMS software

Entity-Relationship Model A logical representation of the data for an organization or for a business area 3 constructs: Entity - person, place, object, event, concept (often corresponds to a row in a table) Attribute - property or characteristic of an entity (often corresponds to a field in a table) Relationship – link between entities (corresponds to primary key-foreign key equivalencies in related tables) Type vs. instance Entity type: Instructor Entity instance: Leon Chen Entity-Relationship Diagram A graphical representation of entity-relationship model Also called E-R diagram or just ERD STUDENT Name Registers 2

Sample E-R Diagram (Figure 3-1) Attribute Supplier_ID Supplier_Name Supplier_Address 3

A special entity that is also a relationship Relationship symbols Attribute symbols Entity symbols A special entity that is also a relationship Relationship degrees specify number of entity types involved Relationship cardinalities specify how many of each entity type is allowed 8

Entity Person: EMPLOYEE, STUDENT Place: WAREHOUSE, COUNTRY Object: BUILDING, MACHINE Event: SALE, REGISTRATION Concept: ACCOUNT, COURSE CERTIFICATE EMPLOYEE DEPENDENT Weak entity Strong entity Associative entity

What Should an Entity Be? SHOULD BE: An object that will have many instances in the database An object that will be composed of multiple attributes An object that we are trying to model SHOULD NOT BE: A user of the database system An output of the database system (e.g. a report)

Figure 3-4 Inappropriate entities System user System output Appropriate entities

Attribute Attribute - property or characteristic of an entity type Classifications of attributes: Required vs. Optional Attributes Simple vs. Composite Attribute Single-Valued vs. Multivalued Attribute Stored vs. Derived Attributes Identifier Attributes - keys 5

Required vs. Optional Attributes Example: entity – ONLINE_ACCOUNT Required attributes (Not NULL) Account_ID Password Owner_Name Optional attributes Phone_Number Password_Hint

Figure 3-7 – A composite attribute An attribute broken into component parts 12

Figure 3-8 – Entity with a multivalued attribute (Skill) and derived attribute (Years_Employed) from date employed and current date Multivalued: an employee can have more than one skill 13

Figure 3-19 – An attribute that is both multivalued and composite This is an example of time-stamping. More examples? 37

Identifiers (Keys) Identifier (Key) - An attribute (or combination of attributes) that uniquely identifies individual instances of an entity type Candidate Key – an attribute that could be a key. Examples (for STUDENT, PERSON)? Simple Key versus Composite Key 6

Figure 3-9a Simple key attribute The key is underlined 14

Figure 3-9b Composite key attribute Flight_Number Date 15

Guidelines for Identifiers Will not change in value Will not be null Substitute new, simple keys for long, composite keys ?  Game_Number 7

Relationship Relationship Type vs. Relationship Instance Degree of a relationship Cardinality of a relationship Associative Entity – combination of relationship and entity

Figure 3-11a attribute of the relationship The relationship type is modeled as the diamond and lines between entity types 20

Relationship Instance is between specific entity instances

Degree of a relationship is the number of entity types that participate in it Entities of two different types related to each other One entity related to another of the same entity type Entities of three different types related to each other 8

Cardinality of a Relationship One-to-One Each entity in the relationship will have exactly one related entity One-to-Many An entity on one side of the relationship can have many related entities, but an entity on the other side will have a maximum of one related entity Many-to-Many Entities on both sides of the relationship can have many related entities on the other side

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Note: a relationship can have attributes of its own 24

Cardinality Constraints Cardinality Constraints - the number of instances of one entity that can or must be associated with each instance of another entity Minimum Cardinality. Examples? If zero, then optional If one or more, then mandatory Maximum Cardinality. Examples? The maximum number 29

Cardinalities Mandatory one Mandatory many Optional one Optional many 11 18

Figure 3-16a Basic relationship Mandatory minimum cardinalities – Figure 3-17a 1

Optional cardinalities with unary degree, one-to-one relationship Figure 3-17c Optional cardinalities with unary degree, one-to-one relationship 34

Maximum Cardinality Constraint max cardinality constraint 41

Entities can be related to one another in more than one way 40

Attributes or Relationship? Attributes can be transformed to relationships

Attributes or Relationship? Attributes should be transformed to relationships

Associative Entities It’s an entity – it has attributes AND it’s a relationship – it links entities together When should a relationship with attributes be an associative entity? All relationships for the associative entity should be many The associative entity could have meaning independent of the other entities The associative entity should have at least one or more attributes other than the identifier The associative entity may participate in other relationships other than the entities of the associated relationship Ternary relationships should be converted to associative entities

Figure 3-11b – An associative entity (CERTIFICATE) 21

Bill of materials structure 27

Figure 3-18 – Ternary relationship as an associative entity 36

Strong vs. Weak Entities Strong entities exist independently of other types of entities has its own unique identifier represented with single-line rectangle Weak entity dependent on a strong entity…cannot exist on its own does not have a unique identifier represented with double-line rectangle Identifying relationship links strong entities to weak entities represented with double line diamond

Strong entity Identifying relationship Weak entity

Figure 3-22a E-R diagram for Pine Valley Furniture

Microsoft Visio Notation for Pine Valley Furniture Different modeling software tools may have different notation for the same constructs

A special entity that is also a relationship Relationship symbols Attribute symbols Entity symbols A special entity that is also a relationship Relationship degrees specify number of entity types involved Relationship cardinalities specify how many of each entity type is allowed 8

Sample E-R Diagram (Figure 3-1) Attribute Supplier_ID Supplier_Name Supplier_Address 3