数据库管理系统概述英文版课件：2 ER Diagram.ppt

COMP231 * One-to-one relationship One-to-one (1-to-1) relationship An entity in A is related to at most one entity in B An entity in B is related to at most one entity in A. 1-to-1 A B COMP231 * One-to-one relationship A customer is associated with at most one loan via the relationship borrower A loan is associated with at most one customer via borrower 1-to-1 A B Key = {customer-id, loan-number} Not a candidate key! Candidate key = ? COMP231 * Many-to-many Relationship Many-to-many Relationship An entity in A is associated with any number of entities in B An entity in B is associated with any number of entities in A That is, there is no restriction in the mapping. Many-to-Many A B COMP231 * Many-to-many Relationship A customer is associated with several (possibly 0) loans via borrower A loan is associated with several (possibly 0) customers via borrower Many-to-Many A B Key = {customer-id, loan-number} Candidate key! COMP231 * Participation Constraint The above constraints (e.g., 1-to-many) tells us that a customer borrows some loans. A natural question to ask is to whether every loan is borrowed by at least one customer. Suppose that each loan is borrowed by at least one customer. Such a constraint is called a participation constraint. 1-to Many A B COMP231 * Participation Constraint We can classify participation in relationships as follows. Total Each entity in the entity set must be associated in at least one relationship Partial Each entity in the entity set may (or may not) be associated in a relationship 1-to Many A B COMP231 * Participation Constraint The participation of loan in borrower is total Every loan must be borrowed by a customer The participation of customer in borrow is partial Some customers may (or may not) borrow loans COMP231 * Outline Entity Relationship Binary relationship Weak Entity/Strong Entity Class Hierarchy Relationship Non-Binary relationship COMP231 * Weak Entities Strong Entity An entity can be uniquely identified by some at