【導(dǎo)讀】refertoFig.電信網(wǎng)管系統(tǒng)示意圖。參照軟件工程中軟件開發(fā)瀑布模型原理，DBAS的生命周。根據(jù)DBAS的軟件組成和各自功能，分為數(shù)據(jù)組織與存儲。根據(jù)數(shù)據(jù)庫系統(tǒng)三級模式結(jié)構(gòu)，DBAS設(shè)計(jì)階段分為概

　　

【正文】 C }among entity sets A, B , C, . R ? A ╳ B ╳ C, replace R by an entity set E = { ei } and three binary relationship set Ra, Rb, Rc Binary vs Nary Relationship Sets R B C A E Rb B Ra A Rc C (b) (a) AttrR AttrR e Nonbinary relationship set translating March 2020 Database System Concepts Chapter6 Database Design and ER Model 69 ? E = { ei } , | E |= | R |, . each (ai , bi , ci ) in R corresponds to one ei in E, or E = { ei }=R={(ai , bi , ci ) } ? Ra = { (ei , ai) | ei ∈ E , ai ∈ A }, relating E and A ? Rb = { (ei , bi) | ei ∈ E , bi ∈ B } , relating E and B ? Rc = { (ei , ci) | ei ∈ E , ci ∈ B }, relating E and C ? E has an identifying attribute e ( candidate key) to distinguish each ei in E ? all attributes of R, . attrR, are assigned to E ? refer to (a) , (b) ? Restriction ? there may not be a way to translating the constraints on a nonbinary relationship into that on several binary relationship, . manytoone Binary vs Nary Relationship Sets (cont.) March 2020 Database System Concepts Chapter6 Database Design and ER Model 70 ? The attributes of a relationship sets R may be associated with one of its participating entity sets ? For the attribute attrA in relationship set R between E1 and E2 (R ? E1 ╳ E2 , if the cardinality of R is ? onetoone : attrA can be assigned as the attribute of either E1 or E2, rather than relationship set R ? onetomany from E1 to E2 : attrA can only be assigned as the attribute of the entity set E2 (the manyside), rather than the relationship set R Placement of Relationship Attributes March 2020 Database System Concepts Chapter6 Database Design and ER Model 71 ? . the relationship depositor in from account to customer is many to one, the attribute accessdate of depositor can be made as an attribute of account, instead of the attribute of the relationship depositor ? manytomany: attrA may only be assigned as the attribute of R, not of its participating entity sets E1 or E2 ? Placement of Relationship Attributes Accessdate as attribute of the account entity set many one Accessdate as attribute of the depositor relationship set many many March 2020 Database System Concepts Chapter6 Database Design and ER Model 74 167。 Weak Entity Sets ? An entity set that does not have a primary key is referred to as a weak entity set ? . payment (number, date, amount) in in next slide, and Fig. ? A weak entity set E1 can only be distinguished through another (strong) entity set E2, called identifying/owner entity set, which has association/ relationship R with E1 ? E2 owns E1 , E1 must be related to E2 via an instance of R, called the identifying relationship. ? R is manytoone ( or onetoone) from the weak entity set E1 to the identity set E2 March 2020 Database System Concepts Chapter6 Database Design and ER Model 75 Weak Entity Sets (cont.) ? E1 is of total participation in the relationship set R ? R is depicted using a double diamond ? weak entity set is depicted as double rectangles. ER diagram with a weak entity set many one total E1 E2 loan1 (1, $10) loan2 (2, $10) payment(1, 2020/02/21, $3) payment(2, 2020/03/21, $3) payment(3, 2020/04/21, $4) payment(1, 2020/02/21, $3) payment(2, 2020/03/21, $3) payment(3, 2020/04/21, $4) one many A example of weak entity set E2 E1 Loan3(3, 20) March 2020 Database System Concepts Chapter6 Database Design and ER Model 77 ? The discriminator (or partial key) of a weak entity set is the set of attributes that distinguishes among all those entities in the weak entity set that depending on a particular strong entity ? . discriminator paymentnumber for weak entity set payment, which depends on loan ? the discriminator of a weak entity set is underlined with a dashed line Weak Entity Sets (cont.) March 2020 Database System Concepts Chapter6 Database Design and ER Model 78 ? The primary key of a weak entity set is formed by the primary key of the strong entity set on which the weak entity set is existence dependent, plus the weak entity set’s discriminator ? primary key for weak entity set E1 = discriminator ∪ primary_key(E2) . ? . discriminator of the payment entity set: paymentnumber, primary key for payment: (loannumber, paymentnumber) ? Weak entity set E1 with several identifying entity sets E2, E3, .., En, primary key for weak entity set E1 = discriminator ∪ primary_key(E2) ∪ … ∪ primary_key(En) . Weak Entity Sets (cont.) March 2020 Database System Concepts Chapter6 Database Design and ER Model 79 ? Another example, in a university, ? a strong entity set ? course(cnumber, title, credits, syllabus, prerequisites) ? a weak entity ? courseoffering(year, semester, instructor(s), section number, timing, classroom) ? the discriminator of courseoffering would be semester (including year) and sectionnumber (if there is more than one section) ? if we model courseoffering as a strong entity we would model coursenumber as an attribute ? then the relationship with course would be implicit in the coursenumber attribute Weak Entity Sets (cont.) March 2020 Database System Concepts Chapter6 Database Design and ER Model 80 ? Other identifying method for the weak entity set E1 ? E1 can be expressed as a multivalued and posite attribute of the identifying entity set E2, refer to Appendix B Weak Entity Sets (cont.) March 2020 Database System Concepts Chapter6 Database Design and ER Model 81 ? Objectoriented (OO) ER ? specialization (特化，特殊化，例化） ? generalization (概括化，泛化，普遍化 ) ? attributes inheritance (屬性繼承） Specialization ? Specialization ? topdown design process of designating subgroups w