CS34311 The Entity- Relationship Model Part I.

CS34312 Database Design Stages Application Requirements Conceptual Design Logical Design Physical Design Conceptual Schema Logical Schema Physical Schema

CS34313 Conceptual Design What is Conceptual Design? Concise representation of our DB application requirements Why Conceptual Design ? It helps us to understand application requirements better It helps us to communicate our understanding of application It helps us to come up with a ‘good’ design

CS34314 Conceptual Design Conceptual Models ER (Entity Relationship) Model, UML (Unified Modeling Language), ORM (Object Role Modeling), etc ER Model Structures: entities and relationships Constraints An ER schema is represented as an ER diagram.

CS34315 ER: Entity Types and Attributes Entity: “Object” Entity Type: “Class” Attribute: property of an entity, has a domain In ER diagrams Entity Type  rectangle Attribute  Oval. Entity Type Student with attributes (sNumber, sName, sAge)

CS34316 ER Example Consider DB instance with 3 students : (1, Joe, 21), (2, Mary, 20), (3, Emily, 20)

CS34317 ER Model: Complex Attributes Composite Attribute: addressMultivalued Attribute: major Student entity type with all its attributes

CS34318 ER Model: Relationship Types Relationship: Association between entities Relationship Type: Class of relationships Representation: Use a diamond shape Relationship type HasTaken to represent Courses taken by Students

CS34319 ER Model: Relationship Types with Attributes Relationship HasTaken has an attribute project which is the project the Student did for the Course

CS Example: Relationship Instances students {Hong, Song}, courses {DB1, DB2}, and relationships {(Hong, DB1 : 98), (Song, DB1 : 99), (Hong, DB2 : 97)}

CS Example : Relationship types Example : Suppliers have a name and an address. Products have a type and a default price. Consumers have a name and a telephone number. Some Suppliers have established contracts to supply a certain Product to a particular Consumer for specially negotiated price at a given quantity. How would you model this ?

CS More relationship types Model the relationship Supplier supplies Products to Consumers Could we make two binary (or three binary) relationships instead?

CS Binary vs. Ternary Relationships What about following binary relationships : S “can-supply” P, C “needs” P, and C “deals-with” S No combination of binary relationships is an adequate substitute: Together 3 binary relationships don’t imply that C has agreed to buy P from S. Also, how could we record qty and price?

CS Recursive Relationship Types Model: All parts have a part number and name. Some parts contain other parts as subcomponents, with a certain quantity. For instance: Both bikes and wheels are parts that we sell. Yet, a bike contains two wheels.

CS Recursive Relationship Types and Roles Recursive relationship type : Part-Subpart Roles: There are Parts that play the role of superPart There are Parts that play the role of subPart

CS ER Model so far Structures Entity Types Relationship Types Binary, ternary, n-ary Recursive (roles) Attributes For entity types and relationship types Simple, composite, multi-valued Roles