Chapter Two Entity-Relationship Data Modeling: Tools and Techniques

CHAPTER 2 ENTITY-RELATIONSHIP DATA MODELING: TOOLS AND TECHNIQUES

▶ ANSWERS TO GROUP I QUESTIONS

2.1 Explain the word schema in your own words. A schema is a description or representation of something that is real or imaginary. A schema is similar to a model in that is describes the way something is constructed.

2.2 Name and explain the purpose of the three schemas in the ANSI/SPARC three schema model. The external schema, sometimes called the user view, is a representation of how users view the database. For all but the simplest databases, an external schema portrays just a portion of the database. A conceptual schema is a complete logical view of the database that contains a description of all the data and relationships in the database. The conceptual schema is logical, in that it is independent of any particular means of storing the data. An internal schema is a representation of a conceptual schema as physically stored using a particular product and/or technique. The description of a set of tables, keys, foreign keys, indexes, and other physical structures is an internal schema.

2.3 Explain a circumstance under which an organization could have three different external schemas for the same conceptual schema. At Lakeview, there might be one external schema for sales, one for marketing, one for operations, and so forth. Or for a university a conceptual schema for a student database could have an external schema for the president of the university, an external schema for a faculty member, and an external schema for a student.

2.4 Describe how external schemas can be used to devise a conceptual schema. External schemas are descriptions of the world as end users see the world. During the database design process, designers collect information from end users and develop the different external schemas for the different end user groups. These different external schemas are then combined into one conceptual schema.

2.5 Why is it important not to confuse the conceptual schema with the internal schema? We do not want to mix the logical view of the data with the physical view. If we do, we may find that the characteristics of the DBMS will constrain and influence our thinking about the conceptual model. In this case, we may miss something important in the user’s world—or, even worse, cause the characteristics of a DBMS to constrain what the users can do. Such constraints allow the tail to wag the dog. We do not want the characteristics of DBMS indexes to determine how salespeople can sell!

2-1 Chapter Two Entity-Relationship Data Modeling: Tools and Techniques

2.6 What was the problem between Bruce and Zelda? Do you think they can ever agree on a single way of viewing the data? The problem, is that Bruce and Zelda have two very different perspectives of the same data. The way they see the world and the way they interact with the world are different. They will probably not agree on a single way of viewing the data. It is Conrad’s job to develop a conceptual schema that can be used to represent both views.

2.7 What is the difference between the E-R model and the extended E-R model?

Subtypes were added to the E-R model to create the extended E-R model. Today, it is the extended E-R model that most people mean when they use the term E-R model.

2.8 Why is IDEF1X important? IDEF1X became a national standard and companies that sold data modeling tools had to conform to IDEF1X in order to sell to the government. Such a large market could not be ignored, so most of today’s popular data modeling products, such as ERWin and Visio, use IDEF1X. This means that IDEF1X is the E-R version you are most likely to encounter, even more so than the extended E-R model.

2.9 Why is UML important? With the growing popularity of object-oriented systems development and object-oriented programming, the use of UML is on the rise.

2.10 Define entity and give an example. An entity is something that can be identified in the users’ work environment; something that the users want to track. Example entities are EMPLOYEE Mary Doe, CUSTOMER 12345, SALES- ORDER 1000, SALESPERSON John Smith, and PRODUCT A4200.

2.11 Explain the difference between an entity class and an entity instance. An entity class is a collection of entities and is described by the structure or format of the entities in that class. An entity instance of an entity class is the representation of a particular entity, such as CUSTOMER 12345; it is described by the values of attributes of the entity. There are usually many instances of an entity in an entity class.

2.12 Define attribute and give examples for the entity you described in question 2.10. Attributes describe the entity’s characteristics. Examples of attributes are EmployeeName, DateOfHire, and JobSkillCode.

2.13 Explain what a composite identifier is and give an example. Identifiers that consist of two or more attributes are called composite identifiers. Examples are {AreaCode, LocalNumber}, {ProjectName, TaskName}, and {CityName and State}.

2.14 Which attribute defined in your answer to question 2.12 identifies the entity? EmployeeName, DateOfHire,

2.15 Define relationship and give an example.

A relationship is an association between two or more entity classes. For example, assume you have an entity class named Student and an entity class named Class. Students Enroll in a Class so you would have a relationship named Enrolls In.

2.16 Explain the difference between a relationship class and a relationship instance. Relationship classes are associations among entity classes, and relationship instances are associations among entity instances.

2.17 Define degree of relationship. Give an example, other than the one in this text, of a relationship greater than degree 2. The number of entity classes in the relationship is the degree of the relationship. Assume entity classes FACULTY, STUDENT, and MAJOR. Now assume a FACULTY is assigned to advise a STUDENT for a given MAJOR.

2.18 List and give an example of the three types of binary relationships. Draw an E-R diagram for each.

FACULTY 1:1 OFFICE

FACULTY-LOCATION

AUTO 1:N REPAIR

AUTO-REPAIR

STUDENT N:M CLASS

STUDENT-SCHEDULE

2.19 Define the terms maximum cardinality and minimum cardinality. Maximum cardinality is the maximum or larges number of entities that can occur on one side of the relationship. Minimum cardinality is the minimum or smallest number of entities that can occur on one side of the relationship.

2.20 Name and sketch the symbols used in the extended E-R model entity-relationship diagrams for (a) entity, (b) relationship, (c) weak entity and its relationship, (d) recursive relationship, and (e) subtype entity.

STUDENT (a) Entity

1:N (b) Relationship

STATE 1:N CITY (c) Weak entity and Relationship

EMPLOYEE 1:1 IS-MARRIED-TO (d) Recursive Relationship

PERSON m    STUDENT STAFF FACULTY (e) Subtype

2.21 Give an example E-R diagram for the entities DEPARTMENT and EMPLOYEE, which have a 1:N relationship. Assume that a DEPARTMENT does not need to have any EMPLOYEE, but that every EMPLOYEE does have a DEPARTMENT.

DEPARTMENT 1:N EMPLOYEE

EMPLOYEE-ASSIGNMENT

2.22 Give an example of a recursive relationship and show it in an E-R diagram. Assume employees are allowed to marry employees. Not all employees are married to employees but if an employee is married, they can one be married to one other employee.

EMPLOYEE 1:1 IS-MARRIED-TO

2.23 Define the term weak entity and give an example other than the one in this text. Weak entities are those that cannot exist in the database unless another type of entity also exists in the database. An entity that is not weak is called a strong entity. Assume that ROOMS cannot exist unless they are in BUILDINGS.

BUILDING 1:N ROOM

BUILDING-CONTAINS

2.24 Explain the ambiguity in the definition of the term weak entity. Explain how this term is interpreted for the extended E-R model. Give examples other than those in this text of each type of weak entity. In a strict sense, if a weak entity is defined as any entity whose presence in the database depends on another entity, then any entity that participates in a relationship having a minimum cardinality of 1 to a second entity is a weak entity. OFFICES must have a BUILDING, CITY must be in a STATE, GRADE must belong to a STUDENT, REPAIR must be made to an AUTO.

2.25 Define the term ID-dependent entity and give an example other than one in this text. An ID-dependent entity is one in which the identifier of one entity includes the identifier of another entity. If you assume that a city can be in only one state but states may have cities with the same name then to identify a city you need to know what state the city is in. For example, there is a Bristol, TN and a Bristol VA.

STATE 1:N CITY

CITY-IS-IN 2.26 Show how to use an ID-dependent entity to represent the multivalued attribute Skill in an EMPLOYEE entity. Indicate both the maximum and minimum cardinalities on both sides of the relationship. Use extended E-R symbols.

EMPLOYEE 1:N SKILL

EMPLOYEE-SKILL

EMPLOYEE Contains: SKILL Contains: Emp Id Skill Emp Name Hire Date Other attributes … 2.27 Show how to use an ID-dependent entity to represent the multi-value composite attribute Phone that contains the single-value attributes AreaCode, PhoneNumber. Assume that Phone appears in an entity called SALESPERSON. Indicate both the maximum and minimum cardinalities on both sides of the relationship. Use extended E-R symbols.

SALESPERSON 1:N PHONE

SALESPERSON-PHONE SALESPERSON Contains: PHONE Contains: Sales Id Area Code Sales Name Phone Number Sales Title Other attributes …

2.28 Describe subtype entities and give an example other than those in this text.

A subtype entity is an entity that represents a special case of another entity, called its supertype. For example, the entity STUDENT could be modeled to have subtypes of GRADUATE_STUDENT and UNDERGRADUATE_STUDENT. Or, STUDENT could be modeled to have subtypes BUSINESS_MAJOR, SCIENCE_MAJOR, LIBERAL_ARTS_MAJOR, and so forth. The entity BUILDING could be modeled to have subtypes of CLASSROOM, OFFICE, or RECREATIONAL. The entity CONGRESSPERSON could be modeled to have subtypes of SENATOR or REPRESENTATIVE.

2.29 Explain the statement “In database processing, subtypes are used when values are inappropriate for some attributes of some entity instances.” Show how this statement applies to your answer to question 2.28. By inappropriate we mean that the attribute does not apply to certain instances of the entity. For example, if a building is an OFFICE building or a RECREATIONAL building then the attribute of Number_of_Desks would not apply but it would apply if the building were a CLASSROOM building. If a building were a RECREATIONAL building then the attribute Gym_Size would apply but it would not apply to an OFFICE or a CLASSROOM building.

2.30 Explain the difference between a HAS-A relationship and an IS-A relationship, and give an example of each. The relationship between a supertype and its subtypes is sometimes called an IS-A relationship. Entities with an IS-A relationship should have the same identifier because they represent different aspects of the same thing. Entities with an HAS-A relationships represent aspects of different things and thus have different identifiers. These relationships do not involve subtypes. BUILDINGs with an identifier of Building Code and subtypes of CLASSROOM, OFFICE, and RECREATIONAL have an IS-A relationship because all types of building are identified by the building code. In essence, all subtypes IS-A BUILDING. A binary relationship between ADVISOR and STUDENT would be a HAS-A relationship because a STUDENT HAS-A ADVISOR, ADVSIOR is not a type of STUDENT.

2.31 Define primary key. The Primary key as the principal unique identifier of the entity.

2.32 What is a foreign key and why are such keys inappropriate in a conceptual model? A foreign key is the Primary key of one table placed in a second table to form a relationship. When creating a conceptual model, one should focus on obtaining a model that supports all of the users’ views and needs for the data. One should not worry about foreign keys and the like. Such keys are part of the internal schema, not the conceptual schema

2.33 What are non-identifying connection relationships? Non-identifying connection relationships are 1:1 or 1:N relationships between two non-ID- dependent (hence non-identifying) entities. Connection relationships are the same as what are called HAS-A relationships in the extended E-R model.

2.34 Give an example of a non-identifying connection relationship.

EMPLOYEE 1:1 IS-MARRIED-TO

In the recursive relationship above the IS-MARRIED-TO relationship is a non-identifying relationship.

2.35 Explain the meaning of a filled-in circle, a diamond, the letter P, the letter Z, and the number 1 in an IDEF1X E-R diagram. a filled-in circle: In IDEF1X, the default cardinality of a non-identifying connection relationship is one-to-many, with a mandatory parent and an optional child. Such relationships are shown by a dashed line with a filled circle by the child, and no other notation. a diamond: If the parent is optional, a diamond is added to the line, adjacent to the parent. the letter P: If the child is required, a P is placed near the circle, indicating that one or more child entities are required (the P stands for positive, as in positive number). the letter Z: a Z indicates that zero or one children are allowed. the number 1: A 1 indicates that exactly one child is required;

2.36 What is the default cardinality of a non-identifying connection relationship? The default cardinality of a non-identifying connection relationship is one-to-many, with a mandatory parent and an optional child.

2.37 How do identifying connection relationships relate to the extended E-R model? Identifying connection relationships are the same as ID-dependent relationships in the extended E- R model.

2.38 What is the difference between a weak entity in the extended E-R model and an identifying connection relationship in the IDEF1X model? In the IDEF1X model, rounded corners signify ID-dependent entities only. There is no way in IDEF1X to document a weak entity that is not an ID-dependent entity. Therefore, and weak entity in the IDE1X model is assumed to have an identifying relationship.

2.39 How does the IDEF1X model represent a weak entity that is not ID-dependent? IDEF1X allows for weak entities that are not identifying, but provides no special notation for such entities. The minimum cardinality for the parent in such relationships is 1, but no other means exists for documenting the fact that they are logically dependent on their parent.

2.40 How are N:M relationships represented in an IDEF1X model? A many-to-many relationship relationship is simply a Non-specific relationship. Non-specific relationships are shown with a filled-in circle on each end of the solid relationship line. In IDEF1X, there is no way to set minimum cardinalities of a non-specific relationship.

2.41 Make the argument that N:M relationships do not exist. Give examples other than the ones in this text in your answer. N:M relationships do not really exist because there is no conceptual data to represent them. They only appear to exist because something has been left out of the model. Such relationships have no direct expression in the relational model. During implementation, they must be converted to two 1:N relationships therefore they do not exist as one N:M relationship but as two 1:N relationships. RIVERS flow through many STATES and STATES and many RIVERS flowing through them. This is true even if a model does not allow you to show the relationship.

2.42 Make the argument that N:M relationships do exist. Give examples other than the ones in this text in your answer. One should be able to fully document an M:N relationship, including minimum cardinalities, in the conceptual model. What happens to that relationship later in designing the internal schema should not be a factor in conceptual design. A Passenger can be on many Flights and a Flight can have many Passengers. However, for a Passenger to board a Flight they must have a Ticket fro each flight. This makes a 1:N relationship between Ticket and Passenger, and a 1:N relationship between Ticket and a Flight.

2.43 Of the two arguments in your answers to question 2.41 and 2.42, which do you believe? I would argue that N:M relationships do exist. First they exist in the user’s view of their world. Secondly, what happens to that relationship later in designing the internal schema should not be a factor in conceptual design.

2.44 Why? Explain the terms generic entity, category entity, and categorization cluster. Give an example of each. A generic entity is similar to a super type in the E-R model. A STUDENT would be generic entity A category entity is similar to a subtype. If a STUDENT could be an UNDERGRADUATE- STUDENT, GRADUATE-STUDENT, or a SPECIAL-STUDENT these would be category entities. A categorization cluster is a group of category entities. In the example above the collection of UNDERGRADUATE-STUDENT, GRADUATE-STUDENT, and SPECIAL-STUDENT entities would be a categorization cluster.

2.45 What is the role of a discriminator in a category relationship? A discriminator is an attribute of the generic entity that indicates the type of the category entity the generic entity is.

2.46 Explain the difference between a complete category cluster and an incomplete category cluster. In a complete category cluster, every possible type of category for the cluster is shown. Complete category clusters are denoted by two horizontal lines, with a gap in-between. In an incomplete category cluster, there may be types of categories that are not shown.

2.47 Explain why the Z notation with complete category clusters is puzzling. The use of the Z notation may appear to be puzzling, if not wrong. As stated, categories in a cluster are mutually exclusive. Thus, after an entity has a relationship to one category entity, the cardinality of the relationship to the other entities in that cluster is zero. Additionally, the Z on the line between the generic entity and the cluster symbol indicates that a generic entity may or may not have a category entity. For complete clusters, the Z should be a 1 in some cases, indicating that the generic entity must have a relationship to a category entity. There is no way to specify a 1 in the IDEF1X model for this case.

2.48 Explain the circumstances under which an entity instance may have a relation-ship to two or more category entity instances. This can occur only is the category entities are in different category clusters. For example a STUDENT may have a relationship to a GRATUATE-STUDENT category entity in one cluster and a relationship to OFF-CAMPUS-STUDENT in a different cluster.

2.49 What are the differences between a categorization relationship in the IDEF1X model and subtypes/supertypes in the extended E-R model? The IDEF1X model adds structure to the generalization/subtype relationships in the extended E-R model. By further defining these concepts, the IDEF1X model makes them more meaningful and hence more useful.

2.50 Explain the normal way that relationships are named. Normally, a name consists of a verb or verb phrase expressed from the standpoint of the parent in the relationship, followed by a slash, and followed by the verb phrase expressed from the standpoint of the child. Normally, the verb phrase from the child’s view is the passive form of the verb phrase from the parent’s view.

2.51 When are relationship names particularly useful? They can be helpful, however, when the relationship between two entities is ambiguous. In Figure 2-25, there are two relationships between CUSTOMER and SALE. One relationship is used to document that a customer can buy something; a second is used to document that a customer can sell something.

2.52 What is a domain? A domain is a named set of values that an attribute can have. A domain can consist of a specific list of values, or it can be defined more generally for example, as a set of strings of maximum length 50.

2.53 Explain two ways that domains reduce ambiguity. Domains can reduce ambiguity when two attributes have the same name but represent different things. For example, different entities may have an attribute of Tax. If this were Sales Tax for one attribute and Income Tax for the other attribute, domains could be defined to distinguish between the two. Furthermore, named domains eliminate ambiguity from attributes whose values look similar, but are not the same.

2.54 Describe how domains are practically useful. One way domains become useful is to assign like attributes to the same domain. For example, if you create a domain for Dates and the domain used a format of MM-DD-YY, all dates assigned to that domain would be forced to use that format. If you decided that the formats for dates need the four-digit year you could change the domain and all dates would now use the new format. Another practical use for domains is to assess whether two attributes that are named differently are referring to the same thing. They would be if they have the same domain.

2.55 Define base domain. A base domain is a domain having a data type and possibly a value list or range definition.

2.56 Define type domain. A type domain is a subset of a base domain or a subset of another type domain. Type domains allow the definition of domain hierarchies that can be used for greater specificity.

▶ ANSWERS TO GROUP II QUESTIONS

Use the STUDENT ACTIVITY IDEF1X diagram in Figure 2-32 to answer the following questions:

2.57 Consider the relationship between STUDENT and EQUIPMENT:

A. What type is this relationship? Non-Identifying Connection Relationship

B. What are the maximum cardinalities? The maximum cardinalities are 1:N. A STUDENT can have many EQUIPMENTs but an EQUIMENT can belong to only one STUDENT.

C. What are the minimum cardinalities? The minimum cardinalities are zero on both sides. An EQUIPMENT does not require a STUDENT as indicated by the diamond. A STUDENT is not required to have an EQUIPMENT indicated by using the default.

D. Describe any cardinality changes that you think should be made. Justify your changes. No changes. Since Students are no required to take course they would not be required to be assigned equipment. It is also possible that a course does not have students so the equipment may not be assigned to a student.

E. Give an appropriate name for this relationship. Assigned/Assigned To

2.58 Consider the relationship between EQUIPMENT and COURSE:

B. What are the maximum cardinalities? The maximum cardinalities are 1:N. A COURSE can have many EQUIPMENTs but an EQUIMENT can belong to only one COURSE.

C. What are the minimum cardinalities? The minimum cardinalities are zero AND 1. An EQUIPMENT must be assigned to a COURSE (default cardinality). A COURSE is not required to have EQUIPMENT indicated by using the default.

D. Describe any cardinality changes that you think should be made. Justify your changes. No changes.

E. Give an appropriate name for this relationship. Uses/Used by

2.59 In Figure 2-32, COURSE describes a course such as Beginning Kayaking or Intermediate Scuba; CLASS describes a particular offering of a course, such as the class Beginning Kayaking offered on January 7, 2003. For the relationship between COURSE and CLASS:

A. What type is this relationship? Is this the correct type for this relationship? Identifying Connection Relationships since the key of the class would include the key of the course (course name). YES, the correct type of relationship is shown.

B. What are the maximum cardinalities? The maximum cardinalities are 1:N. A COURSE can have many CLASSes but a CLASS can belong to only one COURSE.

C. What are the minimum cardinalities? The minimum cardinalities are zero AND 1. A CLASS must be assigned to a COURSE (default cardinality). A COURSE is not required to have CLASSes indicated by using the default.

D. Describe any cardinality changes that you think should be made. Justify your changes. No changes.

E. Give an appropriate name for this relationship. Offers/Offered By

2.60 Consider the relationship between CLASS_FEE and CLASS:

B. What are the maximum cardinalities? The maximum cardinalities are 1:N. A CLASS can have many CLASS_FEEs but a CLASS_FEE can belong to only one CLASS.

C. What are the minimum cardinalities? The minimum cardinalities are zero AND 1. A CLASS_FEE must be assigned to a CLASS (default cardinality). A CLASS is not required to have CLASS_FEE indicated by using the default.

D. Describe any cardinality changes that you think should be made. Justify your changes. No changes. Students are not required to have classes so a class may not have a fee. If a student pays a fee it must be assigned to a class.

E. Give an appropriate name for this relationship. Pays/Charges

2.61 Consider the relationship between CLASS_FEE and STUDENT:

A. What type is this relationship?

Non-Identifying Connection Relationship

B. What are the maximum cardinalities? The maximum cardinalities are 1:N. A STUDENT can pay many CLASS_FEEs but a CLASS_FEE can belong to only one CLASS.

C. What are the minimum cardinalities? The minimum cardinalities are zero and zero. A CLASS_FEE is not required to be assigned to a CLASS as indicated by the diamond. A COURSE is not required to have a CLASS_FEE indicated by using the default.

D. Describe any cardinality changes that you think should be made. Justify your changes. Change the minimum cardinality to STUDENT to the default (required) by removing the diamond. If a CLASS_FEE does not require a STUDENT, who pays the fee?.

E. Give an appropriate name for this relationship. Pays/Paid By

2.62 Consider the relationship between STUDENT and CLUB:

A. What type is this relationship? Non-specific Relationship

B. Describe a possible new entity that would change this relationship into two non- identifying connection relationships. Specify appropriate cardinalities for your new relationships. MEMBERSHIP: This entity would include attributes such as Member_Id, Date_joined, and Dues_Paid. The cardinalities would be 1:M between CLUB and MEMBERSHIP (Clubs can have zero to many Memberships but a Membership must be for a single Club). 1:N between STUDENT and MEMBERSHIP (Students can have zero to many Memberships but each Membership must belong to exactly one Student.

C. Do you think the design in your answer to question B is a better design than the relationship shown in this figure? YES, there are attributes that do not belong to either the CLUB or the STUDENT (Member_Id, Date_Joined, and Dues_Paid. This design allows you to show a cleared design.

2.63 Add a new entity called INSTRUCTOR. Create a new relationship between INSTRUCTOR and COURSE and a new relationship between INSTRUCTOR and EQUIPMENT.

A. Justify the type of relationships you have chosen. Non-Identifying Connection Relationship. Both relationships would be 1:N. They are non- identifying because Instructor, Equipment, and Course have their own identifiers. Note that these relationships would probably make the relationship between EQUIPMENT and COURSE unnecessary.

B. Specify cardinalities for your new relationships.

Both relationships would be 1:N. An Instructor could teach from zero to many Courses but a Course must have one Instructor. An instructor may have from zero to many Equipments (zero if they do not teach a course, many if they teach at least one course) but Equipment must be assigned to only one Instructor since the Equipment must be assigned to one Course and the Course must have one Instructor.

C. Name your new relationships. Between INSTRUCTOR and EQUIPMENT: Uses-Used By Between INSTRUCTOR and COURSE: Teaches-Taught By

D. Redraw the diagram in Figure 2-32 to show your new relationships. Place all cardinalities from your answers to questions 2.57 to 2.62 on this diagram.

CLUB STUDENT EQUIPMENT

INSTRUCTOR

CLASS_FEE COURSE

2.64 Modify your answer in question 2.63 to make INSTRUCTOR and CLUB_OFFICER category entities of STUDENT.

A. Should you use a complete or an incomplete category cluster? Explain your answer. Incomplete Category Cluster. Students are not required to members of a Club so there will be Students that are not Club_Officers. We must assume that not all Students are Instructors to there will be Students that are neither Instructors nor Club_Officers.

B. Name and describe an attribute that could be used as a discriminator for this category cluster. Student_Status. This would be a code to indicate that the Student is an Officer (value O) or Instructor (value I). If the Student is neither the value would be null.

C. Redraw the diagram in your answer to question 2.63 to show your new relationships.

CLASS_FEE CLUB STUDENT CLASS Z Z Z CLUB_OFFICER INSTRUCTOR EQUIPMENT

COURSE

Use the EMPLOYEE TRIP IDEF1X diagram in Figure 2-33 to answer the following questions:

2.65 Consider the relationship between EMLOYEE and TRIP:

B. What are the maximum cardinalities? The maximum cardinalities are 1:N. An EMPLOYEE can have many TRIPs but a TRIP can belong to only one EMPLOYEE.

C. What are the minimum cardinalities? The minimum cardinalities are zero and zero. A TRIP is not required be assigned to an EMPLOYEE as indicated by the diamond. AN EMPLOYEE is not required to have TRIP, indicated by using the default.

D. Describe any cardinality changes that you think should be made. Justify your changes. Change the minimum cardinality at the Employee to mandatory by removing the diamond. If the Trip does not belong to an Employee, who is taking the trip?

E. Give an appropriate name for this relationship. Takes/Taken By

2.66 Consider the relationship between TRIP and AIRLINE_RESERVATION:

B. What are the maximum cardinalities? The maximum cardinalities are 1:N. A TRIP can require many AIRLINE_RESERVATIONSs but an AIRLINE_RESERVATION can belong to only one TRIP.

C. What are the minimum cardinalities?

The minimum cardinalities are zero and zero. An AIRLINE_RESERVATION does not need to be assigned to a TRIP as indicated by the diamond. A TRIP is not required to have an AIRLINE_RESERVATION indicated by using the default.

D. Describe any cardinality changes that you think should be made. Justify your changes. Change the minimum cardinality at the Trip to mandatory by removing the diamond. If the Airline Reservation does not belong to a Trip, why do we even have a reservation?

E. Give an appropriate name for this relationship. Requires/Reserved For

2.67 Consider the relationship between TRIP and CAR_RESERVATION:

B. What are the maximum cardinalities? The maximum cardinalities are 1:N. A TRIP can require many AIRLINE_RESERVATIONSs but an AIRLINE_RESERVATION can belong to only one TRIP.

C. What are the minimum cardinalities? The minimum cardinalities are zero and zero. An AIRLINE_RESERVATION does not need to be assigned to a TRIP as indicated by the diamond. A TRIP is not required to have an AIRLINE_RESERVATION indicated by using the default.

D. Describe any cardinality changes that you think should be made. Justify your changes. Change the minimum cardinality at the Trip to mandatory by removing the diamond. If the Airline Reservation does not belong to a Trip, why do we even have a reservation?

E. Give an appropriate name for this relationship. Requires/Reserved For

2.68 Consider the relationship between TRIP and HOTEL_RESERVATION:

B. What are the maximum cardinalities? The maximum cardinalities are 1:N. A TRIP can require many HOTEL_RESERVATIONSs but an HOTEL_RESERVATION can belong to only one TRIP.

C. What are the minimum cardinalities? The minimum cardinalities are zero and 1. An HOTEL_RESERVATION must be assigned to a TRIP as indicated by the default. A TRIP is not required to have a HOTEL_RESERVATION, indicated by using the default.

D. Describe any cardinality changes that you think should be made. Justify your changes.

No Changes

E. Give an appropriate name for this relationship. May Require/Reserved For

2.69 Consider the relationships among AIRLINE_RESERVATION, E_TICKET, and PAPER_TICKET:

A. What type is this relationship? Complete Categorization Relationship

B. What are the maximum cardinalities? An Airline_Reservation must be either an E_Ticket or a Paper_Ticket but not both. Each E_ticket or Paper_Ticket can be only one Airline_Reservation. This gives a maximum cardinality of 1:1.

C. What are the minimum cardinalities? Since all E_Tickets must be Airline_Reservations but Airline_Reservations are not required to be E_Tickets, the minimum cardinality here is 1 and zero. Since all Paper_Tickets must be Airline_Reservations but Airline_Reservations are not required to be Paper_Tickets, the minimum cardinality here is 1 and zero. Note that since this is a complete Categorization Relationship, the Generic entity Airline_Reservation must have one E_Ticket or one Paper_Ticket so the minimum cardinality between the Airline_Reservation and the category cluster is 1 and 1.

D. Is this a complete or an incomplete category cluster? Should it be a different type of category cluster? It is a complete category cluster. I would argue that this is appropriate. I can think of no other type of ticket required by an airline.

E. Give an example discriminator for this relationship. Ticket_Type: This would be a code carrying one of two values, E for E_Ticket and P for Paper_Ticket.

2.70 Add a new entity called EXPENSE_REPORT. Create a new relationship between EMPLOYEE and EXPENSE_REPORT and a new relationship between TRIP and EXPENSE_REPORT.

A. Justify the type of relationships you have chosen. Both relationships are Non-Identifying Connection Relationships. Each instance of the relationship will be for an expense item for and EMPLOYYE on a TRIP. It is not an Identifying Connection Relationship because each expense item would have its own identifier. It is not a Non-Specific Relationship because it is not a many-to-many relationship. It is not a Categorization Relationship because Expense Reports are not Employees or Trips.

B. Specify cardinalities for your new relationships.

The relationship between Employee and Expense_Report is 1:N. An Employee may have zero instances of Expense_Report since they may not take Trips or they may have many, especially is they take more than one Trip. An Expense_Report instance must belong to an Employee and only one Employee. The relationship between Trip and Expense_Report is also a 1:N. A Trip may have zero instances of Expense_Report since they are not required to have any reservations or they may have many since they can have both airline and hotel reservations. An Expense_Report instance must belong to a Trip and only one Trip.

C. Name your new relationships. Employee and Expense_Report: Submits-Submitted By Trip and Expense_Report: Charges-Charged To

D. Redraw the diagram in Figure 2-33 to show your new relationships.

EMPLOYEE

TRIP EXPENSE_REPORT

HOTEL_RESERVATION CAR_RESERVATION AIRLINE_RESERVATION

E_TICKET PAPER_TICKET

2.71 Modify your answer in question 2.70 to make BUSINESS and PERSONAL category entities of TRIP. Assume that there can be no relationship between EXPENSE_REPORT and PERSONAL.

A. Should you use a complete or an incomplete category cluster? Complete. Expense_Report instances should be either Business or Personnal.

B. Name and describe an attribute that could be used as a discriminator for this category cluster. Expense_Category. The value would be P for Personnal and B for Business.

C. Redraw the diagram in your answer to question 2.70 to show your new relationships.

EMPLOYEE

HOTEL_RESERVATION AIRLINE_RESERVATION

CAR_RESERVATION EXPENSE_REPORT Z Z Z

BUSINESS PERSONNAL E_TICKET PAPER_TICKET

2.72 Consider the relationship between FISH and OBSERVATION:

B. What are the maximum cardinalities? The maximum cardinalities are 1:N. A FISH can require many OBSERVATIONs but an OBSERVATION can belong to only one FISH.

C. What are the minimum cardinalities? The minimum cardinalities are zero and zero. An OBSERVATION may not be for a FISH indicated by the diamond. A FISH is not required to have an OBSERVATION, indicated by using the default.

D. Describe any cardinality changes that you think should be made. Justify your changes. Change the minimum cardinality to the default of one (remove the diamond). Based upon Figure 2-4 of the text, no observation is recorded unless some type of fish is counted.

E. Give an appropriate name for this relationship. Found During/Observed For

2.73 Consider the relationship between RIVER and OBSERVATION:

B. What are the maximum cardinalities?

The maximum cardinalities are 1:N. A RIVER can require many OBSERVATIONs but an OBSERVATION can belong to only one RIVER.

C. What are the minimum cardinalities? The minimum cardinalities are one and one. An OBSERVATION must be for a RIVER indicated by using the default. A RIVER is required to have at least one OBSERVATION, indicated by using the P for positive number.

D. Describe any cardinality changes that you think should be made. Justify your changes. No Changes

E. Give an appropriate name for this relationship. Location of/Recorded at

2.74 Add a new entity called RIVER_SITE as a child entity in an identifying connection relationship to RIVER. Remove the relationship between RIVER and OBSERVATION and place a new relationship between OBSERVATION and RIVER_SITE.

A. Define and justify cardinalities between RIVER and RIVER_SITE. This should be a 1:N relationship. Each River could have several Observation_Sites and an Observation_Site must be located on a River since the relationship is an identifying connection relationship.

B. Define and justify cardinalities between RIVER_SITE and OBSERVATION. This relationship is also a 1:N relationship. Many Observations can be made at a site and all observations must be made at one site.

C. Name your new relationships. River and Observation_Site: Has/Location Of Observation and Observation_Site: Records/Made at

D. Redraw Figure 2-34 given your new design.

FISH RIVER

OBSERVATION RIVER_SITE

2.75 Add a new entity called OBSERVER to your answer to question 2.74. Create a relationship between OBSERVATION and OBSERVER. Also, add a category cluster to OBSERVER with two categories: PROFESSIONAL and VOLUNTEER.

A. What type of relationship is appropriate? Justify your answer.

The relationship between OSERVER and OBSERVATION is a non-identifying connection relationship. It is not an identifying connection relationship because OBSERVATION would have is own independent identifier. It is not a non-specific relationship because it is not a many- to-many relationship. It is not a categorization relationship because Observations are not a type of Observer.

B. Define and justify cardinalities between OBSERVER and OBSERVATION. Name this relationship. Makes-Made By; Cardinalities are 1:N. An Observation must be made by someone (an Observer) and each Observation is made by a single Observer (Bruce or Zelda). An Observer and make many Observations or it may be possible that a new Observer made make zero observations.

C. What type of category cluster is appropriate? Complete: This does assume that all observers will either be Professional or Volunteers.

D. Redraw your answer to question 2.74 given your new design.

FISH OBSERVATION RIVER_SITE RIVER

OBSERVER

PROFESSIONAL VOLUNTEER

2.76 Create a new entity called CLASS. Construct a relationship between VOLUNTEER and CLASS.

A. What type of relationship is appropriate? Justify your answer. It is not a non-specific relationship because it is not a many-to-many relationship. A Volunteer may take many classes and a Class may be taken by many different Volunteers. It is not a non- identifying connection relationship because it is not a 1:N relationship. It is not an identifying connection relationship because Class would have is own independent identifier. It is not a categorization relationship because Classes are not a type of Volunteer.

B. Redraw your answer to question 2.75 given your new relationship.

OBSERVER

PROFESSIONAL VOLUNTEER CLASS

C. Create another entity called CLASS_OFFERING, which represents a particular offering of a particular class. Add and remove relationships as appropriate. For each new relationship, give the type, give the cardinalities, and name the relationship. The relationship between Class and Class_Offering would be an identifying connection relationship with a one to many cardinality. Each Class may have many offerings but any offering is of a single class. The relationship would be named Has-Offered For. The original relationship between Volunteer and Class is replaced by a relationship between Volunteer and Class_Offering. A Volunteer would attend an offering of a class, not all offerings of a class. The relationship would still be a non-specific relationship because it is a many-to-many. The relationship would be name Attends-Attended By.

D. Redraw your answer to question 2.75 given your new design.

OBSERVER CLASS

PROFESSIONAL VOLUNTEER CLASS_OFFERING

2.77 The Public Affairs Office at Highline University receives requests for speakers on particular topics. To be able to respond to such request, that office wants to build a database. In particular, it wants to keep track of topics, speakers, speeches, and organizations to which someone from Highline has spoken.

A. Make a list of possible entities for the Speaker database. TOPICS SPEAKERS SPEECHES ORGANIZATIONS

B. Create an IDEF1X E-R diagram showing only entities (like Figure 2-34). Assume that there is an M:N relationship between SPEAKER and TOPIC.

SPEAKERS SPEECHES

TOPICS ORGANIZATIONS

C. For each relationship in your diagram, specify the relationship type, and the minimum and maximum cardinality on both parent and child. Name each relationship. SPEAKER and TOPIC: This would be a non-specific relationship because it is a many-to-many relationship. SPEAKER and SPEECHES SPEECHES and ORGAINZATIONS

D. Repeat questions B and C, but add an entity to your diagram so that there will be no N:M relationship.

E. Do you prefer the design in question B and C or the design in question D? Justify your preference. 2.78 The Metropolitan Housing Agency (MHA) is a non-profit organization that advocates the development and improvement of low-income housing. The MHA operates in a metropolitan area of approximately 2.2 million people in a midwestern city. The MHA maintains data about the location, availability, and condition of low-income housing in 11 different census tracts in the metropolitan area. Within the boundaries of these tracts are approximately 250 different buildings that provide low-income housing. On average, each building contains 25 apartments or other units. The MHA keeps data about each census tract, including geographic boundaries, median income of the population, elected officials, principal businesses, principal investors involved in attributes in that tract, and other demographic and economic data. It also maintains a limited amount of data about crime. For each building, the MHA stores the name, address, size, owner(s)’s name and address, mortgagor(s)’s name and address, renovations and repairs, and avail-ability of facilities for handicapped people. In addition, the MHA keeps a list of each of the units within each building, including the type of unit, size, number of bedrooms, number of baths, kitchen and dining facilities, location in the building, and any special remarks. The MHA wants to maintain data regarding the average occupancy rates for each unit, but to date it has been unable to collect or store such data. The MHA does, however, keep data about whether a given unit is occupied. The MHA serves as an information clearinghouse and offers three basic ser-vices. First, it works with politicians, lobbyists, and advocacy groups to support legislation that encourages the development of low-income housing through tax incentives, developmental zoning preferences, and other legislative inducements. To accomplish this, the MHA provides information about low- income housing to state, county, and city governments. Second, through speeches, seminars, displays at conventions, and other public relations activities, the MHA officials strive to raise the community’s consciousness about the need for low-income housing. Finally, the MHA provides

information about the availability of low-income housing to other agencies that work with the low- income and homeless populations.

A. Make a list of possible entities for a database to support MHA’s information needs. Elected Officials Principle Businesses Principle Investors Census Tract Building Apartment Crime

B. Construct an IDEF1X entity-relationships diagram for the entities in question A.

OFFICIAL BUSINESS INVESTOR Is_In/Contains Invests_In/Has Represents/Represented by CENSUS TRACT CRIME

Has/Is_committed_In Has/Is_located_In

BUILDING

Contains/Is_In

C. Name each relationship and ensure that you have correctly defined the minimum and maximum cardinalities.

These are shown on the diagram above. Note: You may find that as you answer question B, you will add, remove, or change some of the entities in your answer to question A. This is normal and very typical of the data modeling process. Just keep working back and forth until you have an IDEF1X E-R diagram that you believe is correct.

▶ ANSWERS TO FIREDUP PROJECT QUESTIONS

Consider the situation of FiredUp, which was discussed at the end of Chapter 1. Assume that FiredUp has now developed a line of three different stoves: FiredNow, FiredAlways, and FiredAtCamp. Further, assume that the owners are selling spare parts for each of their stoves and that they also are making stove repairs. Some repairs are at no charge because they are within the stove warranty period; other repairs are

2-23 Chapter Two Entity-Relationship Data Modeling: Tools and Techniques made at a charge for parts only; and still others are made for parts and labor. FiredUp wants to keep track of all of these data. When asked for further details, the owners made the following list: CUSTOMER: Name, StreetAddress, ApartmentNumber, City, State/Province, Zip/PostalCode, Country, EmailAddress, PhoneNumber STOVE: SerialNumber, Type, ManufactureDate, InspectorInitials INVOICE: InvoiceNumber, Date, Customer with a list of items and prices that were sold, TotalPrice REPAIR: RepairNumber, Customer, Stove, Description with a list of items that were used in the repair and the charge for them (if any), TotalAmount of the repair PART: Number, Description, Cost, SalesPrice A. Create an extended E-R entity-relationship diagram of a database for FiredUp. Set the minimum and maximum cardinality of the relationships among entities, as you think is appropriate. Explain your rationale for each cardinality value. Use weak entities as appropriate. Do not use subtypes. Name any ID-dependent entities, if any.

CUSTOMER 1:N REPAIR

1:N N:1 N:M

INVOICE 1:N STOVE PART

CUSTOMER may have from zero to many INVOICEs. If they ordered long ago, the invoice may no longer exist. They could have multiple invoices if they have ordered more than one stove at different times. An INVOICE must belong to a CUSTOMER and cannot belong to more than one customer. An INVOICE may be for more than one STOVE but it must be for at least one stove. A STOVE may not be on an INVOICE (it could be in for repair) but it cannot be on more than one invoice (cannot be sold to two different people). A REPAIR must be made for a CUSTOMER and not more than one customer. A Customer may have more than one stove in for repair or they may not have any stoves in for repair. PARTs can be used on more than one REPAIR but do not have to be used on any repair. A repair may require multiple parts or may not require any parts. A STOVE may be REPAIRED more than once or it may never be repaired. A repair must be made on exactly one stove. B. Modify the entity-relationship diagram in your answer to question A by representing INVOICE and REPAIR with appropriate subtypes. Under what circumstances is this design better than the one in your answer to question A? This model is much more restrictive than the one in A. I would probably use this model only if a customer were not allowed to purchase a stove and parts at the same time.

1:N CUSTOMER REPAIR N:M 1 1:N PART   

N:M INVOICE WARRENTY PARTS_ONLY PARTS&LABOR

1   STOVE_INV PARTS_INV

N:1 STOVE 1:N

C. Transform your answer to question B to an IDEF1X diagram. Make changes to subtypes and weak entity relationships as appropriate.

CUSTOMER REPAIR

PART Z Z Z INVOICE WARRENTY PARTS_ONLY PARTS&LABOR

STOVE_INV PARTS_INV

D. Suppose that FiredUp wants to keep track of home, fax, and cell phone numbers as well as multiple e-mail addresses for each of its customers. Modify your model in A to allow for multiple values of PhoneNumber and EmailAddress. You can use either the extended E-R diagram or the IDEF1X diagram, depending on which you prefer.

PHONE EMAIL_ADDRESS

N:1 N:1

CUSTOMER 1:N REPAIR

1:N N:1 N:M

E. Suppose that FiredUp develops different versions of the same stove product. Thus, it develops a FiredNow Version 1 and a FiredNow Version 2, and so on. Although FiredUp could consider each version of a stove to be a new product, it does not want to do that. Modify your entity-relationship diagram from question D as necessary to account for this situation. You can use either extended E-R or IDEF1X format, depending on which you prefer.

PHONE EMAIL_ADDRESS

N:1 N:1

CUSTOMER 1:N REPAIR

1:N N:1 N:M

  

VERSION1 VERSION2 VERSION3

▶ ANSWERS TO TWIGS TREE TRIMMING SERVICE

Consider the situation of Samantha Green, who owns and operates Twigs Tree Trimming Service, described at the end of Chapter 1. Although many of her jobs are one-time operations to remove a tree or stump, others are recurring: trimming a tree or groups of trees every year or every other year. When business is slow, she calls former clients to remind them of her service and of the need to trim their trees on a regular basis. Samantha grinds the branches and trunks that she cuts into small chips using a chip grinder that she tows behind her truck. The chips are thrown into the back of her truck as they are ground. For years, Samantha took these chips to the recycling center, but recently she discovered that she could sell these chips to nurseries and organizations for use as mulch. Even more recently, she discovered that some of her own tree-trimming customers want to buy chips as well. (Yes, it is possible that customers could pay her to cut a tree and grind the branches, and repurchase their own chips as mulch some time later. Samantha’s response to that was, “I love America!”) Samantha keeps the following data: OWNER OwnerName, Phone, Street, City, State, Zip RECURRING_SERVICE MonthsBetweenService, Description, ServiceFee SERVICE DateOfService, Description, AmountBilled, AmountPaid, DateOfPayment CHIP_DELIVERY CustomerName, Phone, Street, City State, Zip, DateDelivered, LoadSize, AmountBilled, AmountPaid A. To begin, assume that all jobs are recurring. Create an IDEF1X E-R diagram for OWNER, RECURRING_SERVICE, and SERVICE. Model the relationship between RECURRING_SERVICE and SERVICE as an ID-dependent entity. State the type of all relationships, give the minimum and maximum cardinalities, and name each relationship.

OWNER SERVICE Phone RECURRING SERVICE DateOfService OwnerName Description Description Street MonthsBetweenService AmountBilled City ServiceFee AmountPaid State DateOfPayment Zip

Owner and Recurring Service: Non-Identifying connection relationship, Owner may have from zero to many Recurring services, Recurring service must belong to exactly one Owner, HasPerformed/PerformedFor Recurring Service and Service: Identifying connection relationship, Recurring Service can have from one to many Services, Service must belong at exactly one Recurring Service, PerformedLast/BelongsTo B. Now, assume that no jobs are recurring. That is, remove RECURRING_SERVICE and model SERVICE as a strong entity. Create an IDEF1X model: state the type of all relationships, give the cardinalities, and name each relationship.

OWNER SERVICE Phone DateOfService OwnerName Description Street AmountBilled City AmountPaid State DateOfPayment Zip

Owner and Service: Non-Identifying connection relationship, Owner may have from zero to many Services, Service must belong to exactly one Owner, HasPerformed/PerformedFor C. Combine your answers to questions A and B by naming the entity for services that are recurring as RECURRING_SERVICE, naming the entity for services that are non- recurring as NON_RECURRING_SERVICE, and creating a new generic entity called SERVICE that has RECURRING_SERVICE and NON_RECURRING_SERVICE as category entities. Create an IDEF1X model: state the type of all relationships, give the cardinalities, and name each relationship.

OWNER SERVICE Phone DateOfService OwnerName AmountBilled Street AmountPaid City DateOfPayment State Zip Z

NON_RECURRING_SERVICE RECURRING SERVICE

Description Description MonthsBetweenService ServiceFee

Owner and Service: Non-Identifying connection relationship, Owner may have from zero to many Services, Service must belong to exactly one Owner, HasPerformed/PerformedFor Service, Non-Recurring_Service, Recurring_Service: Categorization Relationship, A Service must be either Non_recurring or Recurring but not both, each Non_Recurring and Recurring subtype must belong to exactly one service. D. Add an entity called CHIP_DELIVERY to your model in question C. To begin, assume that none of Samantha’s trimming customers ever buy chips. Create a new entity called CHIP_CUSTOMER for chip deliveries. Create an IDEF1X model: state the type of all relationships, give the cardinalities, and name each relationship.

Z Z CHIP_DELIVERY Phone NON_RECURRING_SERVICE RECURRING SERVICE CustomerName Street Description Description City MonthsBetweenService State ServiceFee Zip DateDelivered LoadSize AmountBilled AmounPaid

In this case there would be no new relationships. Chip_Delivery is not related to Customer because Customers do not buy chips. Chip_Delivery is not a category entity because it is not a Service. The primary reason for not making Chip_Delivery a Service is that a service must belong to a Customer and the Chip_Delivery does not. E. Answer question D, but assume that some of her trimming customers can also buy chips. Use category entities to model the types of customers similar to that in question C. Create an IDEF1X model: state the type of all relationships, give the cardinalities, and name each relationship.

Z Z Z CHIP_CUSTOMER NON_RECURRING_SERVICE RECURRING SERVICE

DateDelivered Description LoadSize Description MonthsBetweenService AmountBilled ServiceFee AmounPaid

In this case I would use only one category entity and use an incomplete categorization relationship. The rationale is that not all Customers buy Chips but there are no additional attributes about those that do not buy chips. All Chip_Customers must be Customers but not all Customers are Chip_Buyers. All other relationships are the same as those in D above.