12.Entity Relationship Model教程.PPTVIP

Entity Relationship Model Entity-Relationship Model An entity is a collection of distinguishable real-world objects that have many common properties. Examples: Students, Instructors, Courses Student entities have properties: name, address, major, graduation-year A student may be “John Smith”, “22 Sage Rd.”, “Computer Science”, “2000” An attribute is a data item that describes a property of an entity Chen’s model,Crow’s Foot model,UML，IDEF1x Entities ER diagram of entities and their attributes in UML Mapping Entities to Relations Each entity in an E-R model is mapped to a separate relation the primary identifier is mapped to the primary key (underlined!) all regular attributes are mapped to an attribute in the table each subpart of a composite attribute is mapped to a different attribute each multi-valued attribute is mapped to a separate relation that inherits the primary key of the parent relation Mapping Entities Relationships Given a set of entities E1,E2,…,Ek, a relationship R defines a rule of correspondence between these entities. An instance r(e1,e2,…,ek) of the R relation means entities e1,e2,…,ek are in a relation r at this instance. If two people are married, they are in a relationship: married(Bob, Margaret) If a student A takes a course C offered by professor B, then A,B,C are in a relationship. Study(A,C,B) Examples Cardinalities of Relationships Min/Max Cardinalities Cardinalities of a relationship R for an entity E are，for one entity instance e: min-card(E, R) : the minimum number of entity instances in F that should be mapped via R max-card(E, R): the maximum number of entity instances in F that can be mapped via R Min/Max Cardinalities(Contd.) If all dots in the entity E have AT MOST one line coming out, we say: max-card(E, R) = 1. ? If more than one line out is possible, we say: max-card(E, R) = N. ? If all dots in the entity E have AT LEAST one line coming out, we say: min-card(E, R) =