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F- Chapter 3 Data Modeling Usrng the Entity-Relationship (ER) Model Mathematically, an irttribute A of entity type E rvhose value set is V can be clef-ined a. a function from E to the power set6 P(V) of V: A: E --> P(V) We refer to the value of attribute A fbr entity e as A(e). The previous definition cor - ers both single-vah"red and multivalued attributes, as well as NULLs. A NULL value i. represented by the entpty ser. For single-varlued attributes, A(e) is restricted to beinc a singleton set for each entitv e in E, whereas there is no restriction on multivaluec attributes.T For a courposite attribute A, the vaiue set V is the Cartesian product or P(yr), P(\/),. . ., P( l.',,), where \1,, y,, . ., {, are the value sets of the simple component attributes that tbrm A: V= P(Vr) x P(y:) x ... x P(y,,) The value set provides all possible values. Usually only a small number of these values exist in the database. Those values represent the data from the state of the miniworld. They correspond to the data as it actually exists in the miniworld. 3.3.3 lnitial Conceptual Design of the COMPANY Database We can now define the entity types for the COMPANY database, based on thc requirements described in Section 3.2. After defining several entity types and their attributes here, we refine our design in Section -1.4 after we introduce the concept of a relationship. According to the requirenlents listed in Section 3.2, rve crrn identif\ four entity types-one correspronding to each of the four items in the specification (see Figure 3.8): ' An entity type DEPARTMENT rvith attribr-rtes Name, Number, Locations. Manager, and Manager_start_date. Locations is the or-rly multivaiued attribute. We can specifl' that both Name and Number are (separate) key attributes because each was specified to be unique. :: An entity type PROJECT with attributes Name, Number, Location, ancl Controlling-department. Iloth Name and Number are (separate) key attribr-rtes. ::, An entity type EMPLOYEE with attributes Name, Ssn, Sex, Address, Salary. Birth_date, Department, and Supervisor. Both Name and Address may be con-rposite attributes; however, <strong>this</strong> wrrs not specified in the requirements. \{e must go back to the users to see i[ any of them rvill ref-er to the individual components of Name-First name, Middle_initial, Last name-or of Address. ' An entity type DEPENDENT with attributes Employee, Dependent_name, Sex. Birth-date, and Relationship (to the employee). 6, The power set P(V) of a set / s the set of alL subsets of V 7 A singleton set s a set wrth only one eLement (value),
)n cov- ,llue is r bcing rr.rlued tluct of 'r)tllpo- csc\ral- cnrini- e on the rti their rcc'pt of idr'ntiff tlcation ,cations, :tribute. t ributes on, irnd l bLltes. ;. Salary, l'tc com- -'l'rts. We diviclual ,oress. rme, Sex, -3. Fname )( Minit )( Lna.e Project ) ( Hours 3.3 Entity Types, Entity Sets, Attributes, and Keys 69 Name ) z--\\I-,z----=\ ( Works on -\(Ssn )l(Salary) )-=: Depadment ) \\\ | / --l EMPLOYEE (Supervisor Birth date Address So f-ar, we have not represented the fact that an employee can work on several projccts, nor have we represented the number of hours per week an employee works on c.rch project. This characteristic is iisted as part of the third requirement in Section -r.1, and it can be represented by a multivalued composite attribute of EMPLOYEE c.rlled Works on with the simple components (Project, Hours). Alternatively, it can be represented as a multivalued composite attribute of PROJECT called Workers with rhe simple components (Employee, Hours). We choose the first alternative in Figure r.8, which shows each of the entity types just described. The Name attribute of EMPLOYEE is shown as a composite attribute, presumably after consultation with thc users. Figure 3.8 Prelrminary design of entity types for the COMPANY database. Some of the shown attributes wiil be ref ined into relationships,
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