Design Approaches Entity relationship Model(covered in this chapter Models an enterprise as a collection of entities and relationships Entity:a“ thing”or“ object” in the enterprise that is distinguishable from other objects Described by a set of attributes Relationship: an association among several entities Represented diagrammatically by an entity-relationship diagram Normalization Theory Formalize what designs are bad, and test for them
Design Approaches • Entity Relationship Model (covered in this chapter) • Models an enterprise as a collection of entities and relationships • Entity: a “thing” or “object” in the enterprise that is distinguishable from other objects • Described by a set of attributes • Relationship: an association among several entities • Represented diagrammatically by an entity-relationship diagram: • Normalization Theory • Formalize what designs are bad, and test for them
E-R Model
E-R Model
ER model --Database Modeling >The eR data mode was developed to facilitate database design by allowing specification of an enterprise schema that represents the overall logical structure of a database >The ER model is very useful in mapping the meanings and interactions of real-world enterprises onto a conceptual schema Because of this usefulness, many database-design tools draw on concepts from the ER model >The eR data model employs three basic concepts entity sets ° relationship sets, attributes >The ER model also has an associated diagrammatic representation the eR diagram, which can express the overall logical structure of a database graphically
ER model -- Database Modeling ➢The ER data mode was developed to facilitate database design by allowing specification of an enterprise schema that represents the overall logical structure of a database. ➢The ER model is very useful in mapping the meanings and interactions of real-world enterprises onto a conceptual schema. Because of this usefulness, many database-design tools draw on concepts from the ER model. ➢The ER data model employs three basic concepts: • entity sets, • relationship sets, • attributes. ➢The ER model also has an associated diagrammatic representation, the ER diagram, which can express the overall logical structure of a database graphically
Entity Sets >An entity is an object that exists and is distinguishable from other objects Example: specific person, company, event, plant P An entity set is a set of entities of the same type that share the same properties Example: set of all persons, companies, trees, holidays >An entity is represented by a set of attributes; i.e descriptive properties possessed by all members of an entity set Example instructor (ID name street sala course=(course id, title, credits) >A subset of the attributes form a primary key of the entity set; i.e., uniquely identifying each member of the set
Entity Sets ➢An entity is an object that exists and is distinguishable from other objects. Example: specific person, company, event, plant ➢An entity set is a set of entities of the same type that share the same properties. Example: set of all persons, companies, trees, holidays ➢An entity is represented by a set of attributes; i.e., descriptive properties possessed by all members of an entity set. Example: instructor = (ID, name, street, city, salary ) course= (course_ id, title, credits) ➢A subset of the attributes form a primary key of the entity set; i.e., uniquely identifying each member of the set
Entity Sets --instructor and student instructor iD instructor name student -ID student name 76766 Crick 98988 Tanaka 45565 Katz 12345Shankar 10101 Srinivasan 00128 Zhang 98345Kim 76543 Brown 76543 Singh 76653Aoi 22222 Einstein 23121 Chavez instructor 44553 Peltier student
Entity Sets -- instructor and student instructor_ID instructor_name student-ID student_name