1 entity-relationship modeling The entity-relationship ( E-R ) model is based on a perception of a real world which consists of a set of basic objects

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1 database systems entity-relationship(er) modeling [02] s. yurttaοs 1

2 1 entity-relationship modeling The entity-relationship ( E-R ) model is based on a perception of a real world which consists of a set of basic objects called entities and relationships among these objects. 1.1 entities and entity sets ffl An entity is an object that exists and is distinguishable from other objects. ffl An entity set is a set of entities of the same type. ffl An entity is represented by a set of attributes. ffl For each attribute there is a set of permitted values, called the domain of that attribute. Formally, an attribute is a function which maps from an entity set into a domain. Thus, every entity is described by a set of (attribute, data value) pairs, one pair for each attribute of the entity set. A database thus includes a collection of entity sets each of which contains any number of entities of the same type. 2

3 1.2 relationships and relationship sets ffl A relationship is an association among several entities. ffl A relationship set is a set of relationships of the same type. Formally,it is a mathematical relation on n 2 (possibly nondistinct) entity sets. If E 1 ;E 2 ;:::;En are entity sets, then a relationship set R is a subset of f(e 1 ;e 2 ;:::;en) j e 1 2 E 1 ;e 2 2 E 2 ;:::;en 2 Eng where (e 1 ;e 2 ;:::;en) is a relationship. ffl Most of the relationship sets in a database system are binary. ffl Occasionally, however, there are relationship sets which involve more than two entity sets. ffl It is always possible to replace a nonbinary relationship set byanumber of distinct binary relationship sets. ffl The function that an entity plays in a relationship is called its role. Roles are normally implicit and are not usually specified. However, they are useful when the meaning of a relationship needs clarification. Such is the case when the entity sets of a relationship set are not distinct. ffl A relationship may also have descriptive attributes. 3

4 ffl branch, the set of all branches of a particular bank. Each branch is described by the attributes branchname, branchcity, and assets. ffl customer, the set of all people who have an account at the bank. Each customer is described by the attributes customername, socialsecurity, street, and customercity. ffl employee, the set of all people who work at the bank. Each employee is described by the attributes employeename and phonenumber. ffl account, the set of all accounts maintained in the bank. Each account is described by the attributes accountnumber and balance. ffl transaction, the set of all account transactions executed in the bank. Each transaction is described by the attributes transactionnumber, date, and amount. 4

5 1.3 ER examples ffl Construct an E-R diagram for a university registrar's office. The office maintains data about each class, including the instructor, the enrollment, and the time and place of the class meetings. For each studentclass pair, a grade is recorded. Document all assumptions that you make about the mapping constraints. Solution: ssn name H H student takes time place H H class ΦΦ dept H H courseno grade enrollment teaches issn name H H instructor This solution assumes that the main entity sets are student, class, and instructor. Obviously there are many other acceptable variations. For example, it is also possible to make place an entity set. The assumptions made are: This E-R diagram cannot model a class meeting at different places at different times, i.e., a class meets only at one particular place and time. There is no guarantee that the database does not have two classes meeting at the same place and time. Each class has a unique instructor. 5

6 ffl Construct an E-R diagram for a car insurance company that has a set of customers, each of whom owns one or more cars. Each car has associated with it zero or any number of recorded accidents. model driver damageamt place ssn address H H name year person owns H H license car log date H H complaintn accident 6

7 ffl We can convert anyweak entity set to strong entity set by simply adding appropriate attributes. Why, then, do we have weak entity sets? Solution: We have weak entities for several reasons: We want to avoid the data duplication and consequent possible inconsistencies caused by duplicating the key of the strong entity. Weak entities reflect the logical structure of an entity being dependent on another entity. Weak entities can be deleted automatically when their strong entity is deleted. Weak entities can be stored physically with their strong entities. 7

8 ffl Define the concept of aggregation and show where this concept is useful. Aggregation is an abstraction through which relationships are treated as higher level entities. Thus the relationship between entities A and B is treated as if it were an entity C. Some examples of this are: Employees work for projects. An employee working for a particular project uses various machinery: name id H H hours employee work uses machinery id number project 8

9 Manufactures have tie-ups with distributors to distribute products. Each tie-up has specified for it the set of products which are to be distributed. name manufacturer tie-up-date name tie-up distribute product name distributor 9

10 ffl Consider an E-R diagram in which the same entity set appears several times. Why is allowing redundancy a bad practice that one should avoid whenever possible? Solution: By using one entity set many times we are missing relationships in the model. For example, in the E-R diagram below: the students taking classes are the same students who are athletes, but in this model will not show that. ssn name H H student takes dept courseno H H class ssn name H H student plays teamname sport 10

11 ffl Consider a university database for the scheduling of classrooms for final exams. The database could be modeled as the single entity set exam, with attributes course-name, section-number, room-number, and time. Alternatively, one or more additional entity sets could be defined, along with relationship sets to replace some of the attributes of the exam entity set, as: course with attributes name, department, and c-number. section with attributes s-number and enrollment, and dependent as aweak entity set on course. room with attributes r-number, capacity, and building. Show an E-R diagram illustrating the use of all three additional entity sets listed. department name H H c-number s-number H H enrollment course ff section of room ff section in H H r-number building capacity ff for exam H H time exam-id 11

12 Explain what application characteristics would influence a decision to include or not include each of the additional entity sets. Solution: The additional entity sets are useful if we wish to store their attributes as part of the database. For the course entity set, we have chosen to include three attributes. If only the primary key(cnumber) were included, and if courses have only one section, then it would be appropriate to replace the course (and section) entity sets by an attribute (c-number) of exam. The reason it is undesirable to have multiple attributes of course as attributes of exam is that it would then be difficult to maintain data on the courses, particularly if a course has no exam or several exams. Similar remarks apply to the room entity set. 12

13 1.4 University Database ffl A university database contains information about professors(identified by social security number, or SSN) and courses(identified by courseid). Professors teach courses; each of the following situations concern the Teaches relationship set. For each situation, draw an ER diagram that describes it(assuming that no further constrains hold). Professors can teach the same course in several semesters, and each offering must be recorded. semesterid ssn Professor Semester N M N Teaches courseid Course 13

14 Professors can teach the same course in several semesters, and only most recent such offering needs to be recorded. (Assume this condition applies in all subsequent questions.) ssn Professor semester N M N Teaches courseid Course 14

15 Every professor must teach some course. ssn Professor semester N M N Teaches courseid Course 15

16 Every professor teaches exactly one course(no more, no less). ssn Professor semester N 1 1 Teaches courseid Course 16

17 Every professor teaches exactly one course(no more, no less), and every course must be taught by some professor. ssn Professor semester N 1 1 Teaches courseid Course 17

18 Now suppose that certain courses can be taught by a team of professors jointly, but it is possible that no one professor in a team can teach the course. Model this situation, introducing additional entity sets and relationship sets if necessary. ssn Professor M N member-of tid Team semester M N Teaches courseid Course 18

19 1.5 Company Database ffl A company database needs to store information about employees (identified by ssn, with salary, and phone as attributes); departments (identified by dno, with dname, and budget as attributes); and children of employees (with name and age as attributes). Employees work in departments; each department is managed by an employee; a child must be identified uniquely by name when the parent (who is an employee; assume that only one parent works for the company) is known. We are not interested in information about a child once the parent leaves the company.) Draw an ER diagram that captures this information. salary ssn H H phone Employees M Dependent 1 Manages N 1 M N Works In dno budget Departments dname H H Child H H name age 19

20 1.6 Specialization and Generalization ffl Specialization and generalization is another aspect of ER modeling. A containment relationship between higher-level and one or more lowerlevel entity sets are created. By Specialization, a high-level entity is decomposed into lower-level entity sets. By Generalization,two or more disjointlower-level entity sets are unionized to compose a new high-level entity set. account-number balance H H account savings-account interest-rate is-a checking-account overdraft-amount 20