Create an ER diagram for each of the following descriptions: (a) Each company operates four departments, and each department belongs to one company. (b) Each department in part (a) employs one or more employees, and each employee works for one department. (c) Each of the employees in part (b) may or may not have one or more dependants, and each dependant belongs to one employee. (d) Each employee in part (c) may or may not have an employment history. (e) Represent all the ER diagrams described in (a), (b), (c), and (d) as a single ER diagram.
11.10 Create an ER diagram for each of the following descriptions:
(a) Each company operates four departments, and each department belongs to one company.
(b) Each department in part (a) employs one or more employees, and each employee works for one
department.
(c) Each of the employees in part (b) may or may not have one or more dependants, and each dependant
belongs to one employee.
(d) Each employee in part (c) may or may not have an employment history.
(e) Represent all the ER diagrams described in (a), (b), (c), and (d) as a single ER diagram.
11.11 You are required to create a conceptual data model of the data requirements for a company that specializes
in IT training. The company has 30 instructors and can handle up to 100 trainees per training session. The
company offers five advanced technology courses, each of which is taught by a teaching team of two or more
instructors. Each instructor is assigned to a maximum of two teaching teams or may be assigned to do
research. Each trainee undertakes one advanced technology course per training session.
(a) Identify the main entity types for the company.
(b) Identify the main relationship types and specify the multiplicity for each relationship. State any assump-
tions you make about the data.
(c) Using your answers for (a) and (b), draw a single ER diagram to represent the data requirements for the
company.
11.12 Read the following case study, which describes the data requirements for a video rental company. The video
rental company has several branches throughout the USA. The data held on each branch is the branch address
made up of street, city, state, and zip code, and the telephone number. Each branch is given a branch number,
which is unique throughout the company. Each branch is allocated staff, which includes a Manager. The
Manager is responsible for the day-to-day running of a given branch. The data held on a member of staff is
his or her name, position, and salary. Each member of staff is given a staff number, which is unique through-
out the company. Each branch has a stock of videos. The data held on a video is the catalog number, video
number, title, category, daily rental, cost, status, and the names of the main actors and the director. The370
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Chapter 11 z Entity–Relationship Modeling
catalog number uniquely identifies each video. However, in most cases, there are several copies of each video
at a branch, and the individual copies are identified using the video number. A video is given a category such
as Action, Adult, Children, Drama, Horror, or Sci-Fi. The status indicates whether a specific copy of a video
is available for rent. Before hiring a video from the company, a customer must first register as a member of
a local branch. The data held on a member is the first and last name, address, and the date that the member
registered at a branch. Each member is given a member number, which is unique throughout all branches of
the company. Once registered, a member is free to rent videos, up to a maximum of ten at any one time. The
data held on each video rented is the rental number, the full name and number of the member, the video num-
ber, title, and daily rental, and the dates the video is rented out and returned. The rental number is unique
throughout the company.
(a) Identify the main entity types of the video rental company.
(b) Identify the main relationship types between the entity types described in (a) and represent each rela-
tionship as an ER diagram.
(c) Determine the multiplicity constraints for each relationships described in (b). Represent the multiplicity
for each relationship in the ER diagrams created in (b).
(d) Identify attributes and associate them with entity or relationship types. Represent each attribute in the ER
diagrams created in (c).
(e) Determine candidate and primary key attributes for each (strong) entity type.
(f) Using your answers (a) to (e) attempt to represent the data requirements of the video rental company as
a single ER diagram. State any assumptions necessary to support your design.
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