ࡱ> 02/M bjbj== WW lrrrrrrrffff r 9 :  $ Z r Jrr JJJrr J JnJ rr ~ OU[f^ 09 i4i JrrrrSemantic objects have attributes that define their characteristics. There are three types of attributes. Simple attributes have a single value. Examples are Invoice number, sales total. Group attributes are composites of other attributes. One example is address. Semantic object attributes are attributes that establish a relationship between one semantic object and another. Look at the following figure which is an example of semantic object diagram, or object diagram. Such diagrams are used by development teams to summarize the structures of objects and to present them visually. Objects are shown in portrait-oriented rectangles. The name of the object appears at the top, and attributes are written in order after the object name. Fig 4-2(a) The DEPARTMENT object contains an example of each of the three types of attributes. Department name, phone number , and FaxPhone Number are simple attributes, each of which represents a single data element. Campus address a is a group attribute containing the simple attributes Building and OfficeNumber, Finally, COLLEGE, PROFESSOR, and STUDENT each are semantic object attributes, which means that those objects are connected to and logically contained in DEPARTMENT. The object attributes , or object links as they are sometimes called, mean that when a user thinks about a DEPARTMENT, he thinks not only about DepartmentName, CampusAddress, PhoneNumber, an FaxPhone Number but also about the COLLEGE, PROFESSORs, and STUDENTs who are related to that department. Since COLLEGE, PROFESSOR and STUDENT also are objects, the complete data model contains object diagrams for them, too. The COLLEGE object contains attributes of the college; the PROFESSOR object contains attributes of the faculty; and the STUDENT object contains attributes of the students. ATTRIBUTE CARDINALITY Each attribute in a semantic object has both a  HYPERLINK "http://www-ec.njit.edu/~sp24" minimum cardinality and a  HYPERLINK "http://www-ec.njit.edu/~sp24" maximum cardinality. Cardinalities are shown as subscripts of attributes in the format N.M, where N is the minimum cardinality and M is the maximum. In the following figure the minimum cardinality of DepartmentName is 1 and the maximum is also 1, which means that exactly one value of the Department Name is required. The cardinality of PhoneNumber is 1.N, meaning that a DEPARTMENT is required to have at least one PhoneNumber but may have many. The cardinality of 0.1 in FaxPhoneNumber means that a DEPARTMENT may have either zero or one FaxPhoneNumber. The cardinalities in groups and attributes in groups can be subtle. Consider the attribute CampusAddress. Its cardinalities are 0.1, meaning a DEPARTMENT need not have an address and has at most one. Now examine attributes inside Campus Address. Both Building and OfficeNumber have cardinalities 1.1. . ou might be wondering how a group can be optional if the attributes in that group are required. The answer is that the cardinalities operate only between the attribute and the container of that attribute. The minimum cardinality of CampusAddress indicates that there need not be a value for address in DEPARTMENT. But the minimum cardinalities of OfficeNumber and Building indicate that both OfficeNumber and Building must exist in CampusAddress. Thus a CampusAddress group need not appear, but if one does, it must have a value for both OfficeNumber and Building. Semantic object model also has object attributes called the  HYPERLINK "http://www-ec.njit.edu/~sp24" paired attributes. h{"X Y % ( . 1 z}"%͸0J5CJ\jV5CJU\j5CJU\jCJU0JCJjCJU jCJU 5CJ\CJ-x ^$a$ / =!"#$%DyK yK :http://www-ec.njit.edu/~sp24DyK yK :http://www-ec.njit.edu/~sp24DyK yK :http://www-ec.njit.edu/~sp24 i0@0 Normal_HmH sH tH 6@6 Heading 1$@& 5CJ\<A@< Default Paragraph Font*B@* Body TextCJ.U@. 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