【正文】 minator of a weak entity set with a dashed line. ? paymentnumber – discriminator of the payment entity set ? Primary key for payment – (loannumber, paymentnumber) 169。Silberschatz, Korth and Sudarshan Database System Concepts Converting NonBinary Relationships to Binary Form ? In general, any nonbinary relationship can be represented using binary relationships by creating an artificial entity set. ? Replace R between entity sets A, B and C by an entity set E, and three relationship sets: 1. RA, relating E and A , relating E and B 3. RC, relating E and C ? Create a special identifying attribute for E ? Add any attributes of R to E ? For each relationship (ai , bi , ci) in R, create 1. a new entity ei in the entity set E 2. add (ei , ai ) to RA 3. add (ei , bi ) to RB 4. add (ei , ci ) to RC 169。Silberschatz, Korth and Sudarshan Database System Concepts Keys for Relationship Sets ? The bination of primary keys of the participating entity sets forms a super key of a relationship set. ? (customerid, accountnumber) is the super key of depositor ? NOTE: this means a pair of entity sets can have at most one relationship in a particular relationship set. ? . if we wish to track all accessdates to each account by each customer, we cannot assume a relationship for each access. We can use a multivalued attribute though ? Must consider the mapping cardinality of the relationship set when deciding the what are the candidate keys ? Need to consider semantics of relationship set in selecting the primary key in case of more than one candidate key 169。Silberschatz, Korth and Sudarshan Database System Concepts ManyToMany Relationship ? A customer is associated with several (possibly 0) loans via borrower ? A loan is associated with several (possibly 0) customers via borrower 169。 they specify how employee entities interact via the worksfor relationship set. ? Roles are indicated in ER diagrams by labeling the lines that connect diamonds to rectangles. ? Role labels are optional, and are used to clarify semantics of the relationship 169。Silberschatz, Korth and Sudarshan Database System Concepts ER Diagrams ? Rectangles represent entity sets. ? Diamonds represent relationship sets. ? Lines link attributes to entity sets and entity sets to relationship sets. ? Ellipses represent attributes ? Double ellipses represent multivalued attributes. ? Dashed ellipses denote derived attributes. ? Underline indicates primary key attributes (will study later) 169。Silberschatz, Korth and Sudarshan Database System Concepts Mapping Cardinalities ? Express the number of entities to which another entity can be associated via a relationship set. ? Most useful in describing binary relationship sets. ? For a binary relationship set the mapping cardinality must be one of the following types: ? One to one ? One to many ? Many to one ? Many to many 169。Silberschatz, Korth and Sudarshan Database System Concepts Relationship Sets ? A relationship is an association among several entities Example: Hayes depositor A102 customer entity relationship set account entity ? A relationship set is a mathematical relation among n ? 2 entities, each taken from entity sets {(e1, e2, … en) | e1 ? E1, e2 ? E2, …, en ? En} where (e1, e2, …, en) is a relationship ? Example: (Hayes, A102) ? depositor 169。Silberschatz, Korth and Sudarshan Database System Concepts Entity Sets ? A database can be modeled as: ? a collection of entities, ? relationship among entities. ? An entity is an object that exists and is distinguishable from other objects. ? Example: specific person, pany, event, plant ? Entities have attributes ? Example: people have names and addresses ? An entity set is a set of entities of the same type that share the same properties. ? Example: set of all persons, panies, trees, holidays 169。Silberschatz, Korth and Sudarshan Database System Concepts Chapter 2: EntityRelationship Model ? Entity Sets ? Relationship Sets ? Design Issues ? Mapping Constraints ? Keys ? ER Diagram ? Extended ER Features ? Design of an ER Database Schema ? Reduction of an ER Schema to Tables 169。Silberschatz, Korth and Sudarshan Database System Concepts Composite Attributes 169。Silberschatz, Korth and Sudarshan Database System Concepts Degree of a Relationship Set ? Refers to number of entity sets that participate in a relationship set. ? Relationship sets that involve two entity sets are binary (or degree two). Generally, most relationship sets in a database system are binary. ? Relationship sets may involve more than two entity sets. ? Relationships between more than two entity sets are rare. Most relationships are binary. (More on this later.) ?. Suppose employees of a bank may have jobs (responsibilities) at multiple branches, with different jobs at different branches. Then there is a ternary relationship set between entity sets employee, job and branch 169。Silberschatz, Korth and Sudarshan Database System Concepts Mapping Cardinalities affect ER Design ? Can make accessdate an attribute of account, instead of a relationship attribute, if each account can have only one customer ? ., the relationship from account to customer is many to one, or equivalently, customer to account is one to many 169。Silberschatz, Korth and Sudarshan Database System Concepts Roles ? Entity sets of a relationship need not be distinct