【正文】 ributes ? customer_name ? A candidate key is the minimal super key ? nonredundant super key ? . customerid is the candidate key of customer ? . accountnumber is the candidate key of account Keys For Entity Sets March 2020 Database System Concepts Chapter6 Database Design and ER Model 32 ? The primary key is a candidate key chosen by the database designer as the principal means of identifying entities within an entity set ? although several candidate keys may exist, one of the candidate keys is selected to be the primary key ? need to consider semantics of relationship set in selecting the primary key in case of more than one candidate key Keys For Entity Sets (cont.) March 2020 Database System Concepts Chapter6 Database Design and ER Model 33 ? Keys for relationship sets R on entities E1, E2, …, En ? R = {(e1, e2, …, e n) | e1∈ E1,…, e n∈ En} ?E1 ╳ E2 ╳ … ╳ En , how to uniquely distinguish each relationship instances {(e1, e2, …, e n) ? ? R is the bination of E1, E2, …, En, each Ei can be uniquely distinguished by primary_key(Ei), 1 ≤i ≤n, so the set of all attributes in primary_key(E1), primary_key(E2), …, primary_key(En) can be used to recognize (e1, e2, …, e n) Keys For Relationship Sets March 2020 Database System Concepts Chapter6 Database Design and ER Model 34 ? The super_key for R ? primary_key(E1) ∪ primary_key(E2) …. ∪ primary_key(En) ? . in , (customerid, accountnumber) is the super key of depositor ? note ? if the attribute names of primarykeys are not unique, the attributes with the same names should be renamed ? The candidate keys for R ? minimal, nonredundant super keys Keys For Relationship Sets (cont.) March 2020 Database System Concepts Chapter6 Database Design and ER Model 35 ? The candidate keys or the primary key for a binary relationship set R among entity sets A and B can be decided as follows, in accordance with the mapping cardinality of R ? R is manytomany, primary_key(R) = primary_key(A) ∪ primary_key(B) ? R is manytoone from A to B, !! primary_key(R) = primary_key(A) ? R is onetomany, !! primary_key(R) = primary_key(B) Keys For Relationship Sets (cont.) March 2020 Database System Concepts Chapter6 Database Design and ER Model 36 ? R is onetoone, primary_key(R) = primary_key(A) or: primary_key(R) = primary_key(B) Keys For Relationship Sets (cont.) March 2020 Database System Concepts Chapter6 Database Design and ER Model 37 ? The participation of an entity E in a relationship R is total, if ? every entity in E participates in at least one relationship in R ? . participation of loan in borrower is total ? refer to ? every loan must have a customer associated to it via borrower ? The participation of an entity E in a relationship R is partial, if ? some entities in E may not participate in any relationship in R ? . participation of customer in borrower is partial ? refer to Fig. 167。 ) ? keys (167。 The EntityRelationship Model March 2020 Database System Concepts Chapter6 Database Design and ER Model 14 ? An entity is represented by a set of attributes, . the descriptive properties possessed by each member of an entity set ? . customer = (customerid, customername, customerstreet, customercity) loan = (loannumber, amount) Entity Sets (cont.) March 2020 Database System Concepts Chapter6 Database Design and ER Model 15 ? A entity consists of all values of its all attributes ? . Fig ? Entityset customerset = { IDvalue, namevalue, street value, city value | IDvalue ∈ D1, namevalue ∈ D2, street value∈ D3, city value D4} ? D1 ╳ D2 ╳ D3 ╳ D4 Entity Sets (cont.) Entity Sets customer and loan customerid customer customer customer loan amount name street city number March 2020 Database System Concepts Chapter6 Database Design and ER Model 17 ? A relationship is an association among several entities ? . in Fig. Jone borrower L17 customer relationship loan entity set entity set ? A Relationship set （聯(lián)系集，聯(lián)系型） is a set of relationship of the same type ? note: more than one relationship set among the same entity sets 167。 ) ? when applying ER model to model the objects in real worlds, some issues (considerations and principles) should be addressed, to guarantee DBS effectiveness and efficiency for data ? Part 4. Reduction to relational schemas (167。 ) ? (167。 ) ? requirement analysis, conceptual design, logical design, physical design ? Part 2. The EntityRelationship Model ? basic ER model ? modeling elements (167。 ): entity sets, relationship sets, attributes ? constraints (167。 ) induce OO features into ER model , that is, specialization, generalization, attributes inheritance, constraints on genelization ? () aggregation: relationship among relationships ? (167。 ) ? mapping elements in ER model to that in relational models, . conceptual schema ?initial logical schema Four Parts in Chapter 6 (cont.) March 2020 Database System Concepts Chapter6 Database Design and ER Model 6 ? Database design consists of two sequential phases ? analyzing of user requirements ? what data should be stored in the database ? what operations/transaction, such as insert, delete, update and retrieve are needed to conducted on these data ? designing of DB schemas , in accordance w