Consider the Company AMAHORO Ltd with departments like Accounting and Finance,
    Human Resource and ICT. Each department has its own employees who are each
    identified by Id, First Name, Surname, Salary, telephone number and Address.

    Suppose that this Company uses the file approach to keep information of its employees,
    where each department has its own file to store necessary information. The payroll file
    is managed in Accounting and Finance departments and a record of an employee’s
    salary is described by Employee Id, First name, Surname and Salary Amount.

    Assume that a new column for the date of recruitment is to be added to the payroll and
    there is a request in Accounting and Finance departments to retrieve all the employees’
    details whose salaries are greater than 100,000 FRW. One employee finds that his/her
    first name was not correctly written on the payroll.

    Since each department has own separate file, it is difficulty to the Human Resource
    Manager to access all the files and make any modification any time he/she wants.

    1. Discuss whether the approach described above is well designed?

    2. Describe the difficulties each department may encounter?

    3. Discuss and Suggest the CEO of this Company the best and efficient
       approach to be used for overcoming all difficulties.


    Sometimes ago, organizations usually stored information in a way which was with
    many risks of being lost, damaged, corrupted and not well organized. It was kept
    inTraditional File Processing (TFP) which presented many disadvantages.

    Nowadays, well-designed database arises to overcome those problems and saves the
    time in the long run for the user. It is facilitated by a Database Management Systems
    (DBMS). The design of such database presents 3 main levels called “Database design
    levels”. These levels help to make easy the understanding and the management of a
    database. Later on, it will not be very difficult for the software developer to use that

    ACTIVITY 5.1

    Read the following scenario and give answers to the asked questions

    Your school needs to keep information securely of its teachers, students, combinations
    and courses. Each teacher is characterized by its identity number, first name, second
    name, salary, qualification, address, telephone number and email.
    The Combination is identified by its Id and Name. The student identified by its Id, First
    Name, Surname, Address, School fees, Contact Number, Combination Id, Class and
    Course is identified by the Course Id, Title, Combination,
    From the above,

    1. Discuss the data types for the attributes identifying entity Teacher,
        Combination, Student and Subject.

    2. Describe the relationships between entities: Teacher, Combination,
        Student and Subject?

    5.1.1 The conceptual level

    The conceptual levelis concerned with concept(abstract), an idea of what something
    is or how it works; something formed in the mind; a mental image.

    There exist different models used for the conceptual level to represent all the data
    elements likely to belong to the database but the most used is the Entity- Relationship
    Model (ERM) It uses the main concepts like entities, attributes and relationships.

    An entity represents a real-world object such as an employee or a project and has
    attributes that represent properties such as EmpId, FirstName, Surname, Address
    and Birthdate. A relationship represents an association or a link among entities.

    For example, when an employee works on many projects, a relationship exists
    between the employee and each project.

    If an employee is identified by EmpId, FirstName, Surname, Address and Birthdate
    while a project is identified by its ProjectId and ProjectName, the diagram
    representing the Entity Relationship is the following.
    5.1.2 The logical level

    The logical level makes it possible to create relational structures enabling us to put
    into practice the conceptualization by imagining a relational Database Management
    System (DBMS). It is characterized by clear and sound reasoning. At this level there
    exist different models like network data models and hierarchical data models and
    relational model. Currently the mostly used is the Relational model. In the case of
    database having Employee and Project entities the relational model is represented
    by the following tables.
    5.1.3 Physical level

    The physical level is concerned with how data will be encoded and stored. It
    consists of the practical application (Database Management Systems - DBMS) of
    all the preceding theories by using computers together with its software to create
    a database. It deals with storage and processing performance, volumetric (time &
    space), partitioning and distribution.

    The most known development tools to use are MS Access, MySQL (My Structured
    Query Language), SQL (Structured Query Language) and NoSQL.
    Application Activity 5.1

    Galleries keep information about artists, their identification, their names, birth address,
    age, and art style. For each piece of artwork/product, the artist, the production year, its
    title, its type of art (e.g., painting, filmmaking, photograph, crafts, sculpture, drawings,
    etc), and its price in FRW must be stored. Pieces of artwork are also classified into
    groups of various kinds, for example, Rwandan art, 19th-century Art(old testament,
    the French art,Haloween,etc), works by Pablo Picasso; a given piece may belong to
    more than one group.Each group is described by an id and name.

    Finally, galleries keep information about customers. For each customer, galleries keep
    that person’s unique identification, full name, and address, total amount of FRW spent
    in the gallery, the artists and groups of art that the customers tend to appreciate.

    Draw conceptual, logical and physical levels for the above described database.
    Example: SCHOOLS Database

    • A View for registrar office
    Course info (coursid:string,enrollment:integer)

    • The conceptual schema:
    Students(studid: string, name: string, login:string, age: integer, gpa:real)

    Courses(coursid: string, ccoursname:string, courcredits:integer)

    Enrolled(sstudid:string, coursid:string, grade:string)

    • The physical schema:

    Relations stored as unordered files.

    Index on first column of Students.


    Learning Activity 5.2.

    Read the following scenario and give answers to the asked questions
    At Groupe Scolaire Kinazi, the management of students’ information is not
    computerized. The Headteacher is looking for a solution so that the database of
    students can be managed digitally and hence speed up the production of transcripts
    and the search of information when they are requested for. If you are given the task to
    design that database, how are you going to proceed?

    1. Discuss the different steps to use so that you can succeed the

    2. Discuss the financial savings made by the school when using
       computerized application instead of manual system.

    The most important thing to do to start designing a database is to think ahead.
    When a case that needs a database creation is presented, before to switch on the
    computer, it is better to think about the type of information to work with and the
    types of questions that the database should answer, what information needs to
    be stored and what specifically are the links between them. The next phases are
    to verify all requirements specifications, to represent the data with diagram, and
    to plan the database, here are three rules of database design: Rule 1-Plan; Rule
    2-Plan; Rule3-Plan.

    When data is more complex, there are more needs to plan. Even the simplest
    database should be thought through on paper before being created in any tool such
    as Microsoft Access, SQL, MySQL, etc.

    A well-designed database performs well and adapts to future needs by giving users
    access to essential information. Poor planning often results in a database that fails

    to meet overlooked needs.

    In planning the database, regardless of its size and complexity the following basic
    steps are used:

    1. Investigate the information.

    2. Identify the objects.

    3. Model the objects.

    4. Identify the types of information for each object.

    5. Identify the relationships between objects.

    6. Database optimization through normalization.

    7. Data entry and manipulation

    5.2.1. Investigate information

    Before creating a database, there is a need of good understanding of the problem
    that the database is expected to solve. If the database is to replace the traditional
    method, file based approach method, then the existing system will give most of the
    information needed.

    During investing information, there is a need to work with everyone involved in the
    existing system to see what is needed from the new database. Gathering techniques
    include collect copies of customer information, management reports, and any other
    documents that are part of the existing system, because these will be useful in
    designing the database and the interfaces.

    5.2.2. Identifying the important entities and their attributes

    During the process of gathering/investigating information, the key objects or
    entities that will be managed by the database must be identified. The object can be
    a tangible thing, such as a person (for example student, employee, and patient) or a
    product, or it can be a more intangible item, such as a department in an institution,
    a Combination in a school. Each distinct item in the database should have a
    corresponding table for which column titles are attributes of the entity

    5.2.3. Identifying the Relationship Between entities

    One of the strengths of an E-R database is the ability to relate or associate information
    about various items in the database.

    Isolated types of information can be stored separately, but the database can combine
    data when it is required. Identifying the relationship between entities in the design
    process requires looking at the entities, determining how they are logically related,
    and adding relational columns that establish a link from one table to another.

    5.2.4. Modeling the objects

    As the objects in the system and their attributes are identified, they are recorded in
    a way that represents the system visually. They are recorded using Relational model
    and Entity Relational model

    5.2.5. Identifying the types of information for each object

    After identifying the primary objects/entities in the database as candidates for
    tables, the next step is to identify the types of information that must be stored for
    each object.

    These are the columns in the table of the object.

    Fields/columns should be kept simple, the more atomic your fields the more flexible
    will be your database.

    For example, in a database of names and addresses, you would keep each part of
    the person’s name as a separate field.

    The columns in a database table contain a few common types of information:

    Raw data columns

    These columns store tangible pieces of information, such as names.

    Categorical columns

    These columns group the data and store a limited selection of data such as true/
    false, married/single/divorced/widowed, Male/Female, etc.

    Identifier columns

    These columns provide a mechanism to identify each item stored in the database
    table. These columns frequently have an id or number in their name, for example,
    EmployeeId, StudentId, PersonIdNo and InvoiceNumber. The identifier column is the
    primary component.

    Relational or referential columns

    These columns establish a link between information in one entity and related
    information in another entity.

    For example, an entity that tracks sales transactions will generally have a link to the
    customer’s entity so that the complete customer information can be associated with
    the sales transaction.

    5.2.6. Database optimization through normalization

    One of the most important step to consider when designing a database is database
    definition. If tables are not set up properly, it can cause a lot of headaches down
    the road the time of extracting/retrieving required data. Understanding the rules
    of normalization enforces redundancy elimination and inconsistent dependency in
    database designs.

    5.2.7. Data entry and manipulation

    The goal of data entry is to create data that are valid and well organized to assure
    their quality during extraction. Well stored data leads to data consistency.

    Application activity 5.2.

    Discuss the benefits of following database design steps?


    Learning Activity 5.3.

    Read the following scenario and give answers to the asked questions
    Suppose that School wants to develop an information system in which student studies
    in one of the combination and learns different subjects. The system will record details
    concerning student, combination and subject.

          1. Which properties would you expect to find in each student object?

          2. Arrange those properties in tabular form of mxn dimension

    5.3.1. Introduction

    The relational data model was introduced by C. F. Codd in 1970. Currently, it is the
    most widely used data model. It describes the world as “a collection of inter-related
    relations (or tables).”

         a. Relation

    A relation, also known as a table or file, is a subset of the Cartesian product of a list
    of domains characterized by a name. You can also think of it this way: an attribute is
    used to define the record and a record contains a set of attributes.

    The steps below outline the logic between a relation and its domains.

    1. Given n domains are denoted by D1, D2 Dn

    2. And r is a relation defined on these domains

    3. hen r C D1×D2×…×Dn

    The following are the key component to know when we are talking about relation:
    Relation, Tuple, Attribute, Cardinality, Degree, Primary key, Domain

    b. Equivalent Database Concepts

    • Relation <=> Table

    • Tuple <=> Row or record

    • Attribute <=> Column or field

    • Cardinality <=> Number of rows

    • Degree <=> Number of columns

    • Primary key <=> Unique identifier

    • Domain <=> Pool of legal values

    c. Table

    Data is stored in rows and columns. Each Row is known as record and the data items
    are known as fields. Tables contain data about one type of item, person or event, for

    • a table of patients, a table of a student, a table of teacher, a table of books and
    a table of doctor’s appointment

    For a book the fields could include:

      • Title, Author, ISBN, Publication house


    A database stores pieces of information or facts in an organized way. The principal
    storage units are called columns or fields or attributes. These house the basic
    components of data into which your content can be broken down.


    Each row represents a group of related data values, such as a customer or an
    employee. A row, or record, is also known as a tuple


    The link/intersection between column and record is known as cell. It also needs to
    be available so that they can be reconstituted into their whole form, the basis of all


    A simple table below gives us the clearest picture of how records and fields work
    together in a database storage project.

     d. Null value

    In many situations every row and column will contain data, but there cases where it
    makes sense for some columns to not contain a value.
    In our example StudentId cannot be null because it is unique but Sex can be null
    because it is optional field

    e. Degree

    The degree is the number of attributes in a table. In our example above in Figure 5.4
    the degree is 6.

    f. Domain

    A domain is the original sets of atomic values used to model data. A domain is a set
    of acceptable values that a column is allowed to contain.

    For example:

    • The domain of Marital Status has a set of possibilities: Married, Single, Divorced,

    • The domain of Shift has the set of all possible days: {Monday, Tuesday,
       Wednesday, Thursday, Friday, Saturday and Sunday}.

    • The domain of Salary is the set of all floating-point numbers greater than 0 and
       less than 200,000 FRW.

    • The domain of First Name and Surname is the set of character strings that
    represents names of people.

    g. Datatype

    Each field in a record has its own data type. The data types in the fields can be text,
    alphanumeric, numeric, and Boolean or date/time.

    Table 5.1: Datatype example

    Application Activity 5.3.

    1. Suppose that a hospital wants to develop system which manages
    doctors and patients.

    Patients are treated in a ward by the doctors assigned to them. Each patient will be
    assigned a single doctor. What fields would you expect to find in each record for a
    hospital patient and doctor

    2. The following table describes information about employees, study it
       and answer the following questions :

    a. Using correct terminology, identify and describe all the components of
        employee table.

    b. What is the possible domain for field EmpId?

    c. How many records are shown?

    d. How many attributes are shown?

    e. Explain the datatype for each field.

    5.3.2 Queries in design view

    Learning activity 5.4.

    Microsoft Access is a database management system from Microsoft that deals with
    relational model and it is consisted of the following different views:

    a. Layout view

    b. Backstage view

    c. Design view

    d. Datasheet view

    Which of the view above contains command buttons that execute operations on entire
    databases? Explain its advantages.

    Data in a Microsoft Access database is stored in various interlinked tables. A database
    also has forms for data entry though you could enter data directly in tables and
    queries for manipulating your data and permitting information to be retrieved from
    a table. It also allows filtering so only the required records and fields are seen.

    Steps for creating database in Microsoft Access

    • Open Microsoft Access

    • Click on Blank database in the task pane

    • Name your database file (say Company) and click the Create button to create
       the database.

    Steps for creating a table in Microsoft Access-Design View

    1. Open an created database in access DBMS

    2. Click on create tab from menu bar

    3. In table group click on table design icon

    4. Fill field names, data type and their descriptions

    Data in an Access table is stored in various fields. Those fields have different properties
    such as fieldname, size and data types as shown in the figure below:

    Steps for creating relationship between created tables

    1. Open a created database in access DBMS

    2. Click on database tools tab from menu bar

    3. In relationships group click on relationships icon then the relationships
         panel will be displayed

    4. Select a table name and click on add button

    5. Repeat step 4 to all table you want to use

    6. Click on design tab from menu bar

    7. In tools panel click on edit relationship icon

    8. On opened window click on create new button

    9. Select the table names and their columns that you want to join

    10. Click on ok button

    11. Repeat 8, 9, and 10 to all pair table to be joined

    After creating a table, enter the data. Note that there are various methods to create
    tables in Access, such as by using Table Wizard, Datasheet view or by importing
    tables, but this Unit restricts only to Design view.

    Steps for creating a query using design view-Query by example

    Creation of a query in design view has three rows, one to sort the fields, the other
    to specify whether or not to display a field, and the last to specify some criteria to
    select the records.

    1. Click on Create tab of menu from bar

    2. In queries grout click on query Design icon

    3. Select tables to use then click on Add button

    4. In Query Type group select a query commend to be used, Example Select

    5. In query panel select the table names and field names you want to select
        in different tables

    6. Check in show row ender the select item if you want to display them on
        output at run time

    7. Rename the created query

    8. In Result group of menu bar click on Run icon

    After the above steps you will get a table containing the selected items as shown in
    the examples below:

    Query 1: Company leader wants to display onlyEmpId, Fullname (sorted is ascending
    order) and salary for all employees.

    1. Select EmpId, EmpId, FullName, Salary and check them

    2. Select Sort by Ascending on FullName


    Query2: The CEO wants to see only EmpId and FullNames where Salary is less or
    equal to 240000.
    Select EmpId, EmpId , FullName and check them
    Set criteria and uncheck Salary field

    Query 3: The following search condition was entered using a query design view
    (Query by example) grid:

    What will be displayed?

    5.3.3 Dynamic queries using parameter

    A parameter is a piece of information you supply to a query right as you run it.

    Creating a parameter query

    Creating a parameter is similar to adding a normal criterion to a query:

    1. Create a select query, and then open the query Design view.

    2. In the Criteria row of the field you want to apply a parameter to, enter the
          text that you want to display in the parameter box, enclosed in square
      For example, [Enter the Doj:]

      3. Repeat step 2 for each field you want to add parameters to.
    When you run the query, the prompt appears without the square brackets.

    Fill in the value you’re looking for, and then click OK.

    Specify parameter data types

    To specify the data type for parameters in a query:

    1. With the query open in Design view, on the Design tab, in the Show/Hide
        group, click Parameters.

    2. In the Query Parameters box, in the Parameter column, enter the prompt
        for each parameter you want to specify a data type for.
        Make sure that each parameter matches the prompt that you used in the Criteria
        row of the query design grid.

    3. In the Data Type column, select the data type for each parameter.

    Application Activity 5.4.

    The database WORLD is composed of one table called WORLDCITIES Figure 5.14.
    which stores information about World Cities. Use it to answer asked questions
    1. The query-design view grid below selects all Cities from America where
    Population is greater than 8 million.

    Figure 5.13 QBE grid to display Name of city located in America having population >8M

    Show what would be the output.

    2. Copy and Complete the query design view grid below to select and
       show Name_of_city where Population is greater than to 13 and Area is
       equal to 1590.


    5.4.1. Introduction

    Learning Activity 5.5.

    A School needs to store information about Teacher (identified by TeacherId, FirstName,
    Surname, Salary, Qualification, Address, Contact); Combination (identified by
    CombinationId, CombinationName),Student (identified by StudentId, FirstName,
    Surname, Address, Schoolfees, ContactNumber) and Subject (identified by subjectId,

    Discribe the logical relationships between objects Teacher-Student, Teacher-
    Combination, Student-Combination and Teacher-Subject??

    The entity relationship (ER) data model has existed for over 35 years. It is well suited
    to data modelling for use with databases because it is fairly abstract and is easy to
    discuss and explain. ER models are readily translated to relations. ER models, also
    called an ER schema, are represented by ER diagrams.
    ER modelling is based on two concepts:

    • Entities, defined as tables that hold specific information (data)

    • Relationships, defined as the associations or interactions between entities

    5.4.2. Entity Relationship Diagram

    The ER diagram is used to represent the conceptual database schema. In ER diagram:
    Entity, Attributes and Relationships form the components of ER Diagram and there
    are defined symbols and shapes to represent each one of them.

    1. Entity

    A rectangle represents an entity set.

    2. Relationships between Entities
        A diamond represents a relationship.

    3. Entity Types

    • Entity types ->boxes

    • Weak entity type -> double box

    Entity types are similar to classes; they describe potential objects (entities) that will
    appear in the database. Weak entity types describe dependent entities, entities that
    depend on other entities for identity.

    4. Attribute

    An ellipse represents an attribute (Property).

    5. Link between attribute and entity set

    Lines represent linking of attributes (properties) to entity sets.

    5.4.4. Entities and entity sets

    An entity set is a set of entities of the same type that share the same properties. A
    noun is used to represent an entity set. An entity is an instance of an entity set.

    For example, an entity can be:

    • Concrete (TEACHER or STUDENT)

    • Insubstantial (GRADE)

    • An occurrence (EXAM)

    5.4.5. Attributes

    A characteristic of an entity, for example First name, Last name and Age. An attribute
    is a data item that describes a property of an entity set. Attributes determine, explain,
    or categorize an entity set. Attributes have values, which can be of different data
    types such as numbers, character strings, dates, images, sounds, and so on. In a
    physical model, an attribute is a named column in a table. Each table has a list of
    attributes (or columns).

    The types of attributes are:

    Simple (atomic) attribute – This type of attribute has a single component. It is called
    single-valued attribute. For example, the Gender attribute has a single component
    with two values.

    In the COMPANY database, an example of this would be: Name = {John} ; Age = {23}
    Composite attribute – A composite attribute consists of many components.

    For example, the Name attribute has the components Last name and First name. So
    this would be written as → Name = {KAGABO+Peter}

    Address may consist of Province and District, Number, Avenue. So this would be
    written as → Address = {KG +‘14’ + ‘Av’}

    Single valued attribute – This type of attribute has one value for one entity. For
    example, the Title attribute has a single value for each teacher.

    Multi-valued attribute – A multi-valued attribute has many values for one entity.

    An example of a multivalued attribute from the COMPANY database, as seen in
    Figure below, are the degrees of an employee: BSc, MSc., PhD.

    Derived attribute – A derived attribute has its value computed from another attribute
    or attributes.

    A derived attribute is not a part of a table from a database, but is shown for clarity or
    included for design purposes even though it adds no semantic information; it also
    provides clues for application programmers.

    An example of this can be seen in Figure below. Age can be derived from the attribute

    In this situation, Birthdate is called a stored attribute, which is physically saved to the

    Unstable attributes - This type of attribute have values that always change. For
    example, the salary of employee.

    Mandatory attributes - Mandatory attributes must have a value. For example, in
    most businesses that track personal information, Name is required.

    Optional attributes - Optional attributes may have a value or be left null; For
    example, address of employee

    Unique identifier - This type of attribute distinguishes one entity from another.

    For example, in a company, you can distinguish between one employee and another
    using a EmployeeID.

    5.4.6. Keys
    An important constraint on an entity is the key. The key is an attribute or a group
    of attributes whose values can be used to uniquely identify an individual entity in
    an entity set. In the case of Logical Model you use a special approach for unique
    identifier.The equivalent concept for unique identifier within Logical Model is a key.

    A key is a field or a set of fields that has/have a unique value for each record in
    the relation. You need a key to ensure that you do not meet redundancies within a
    relation There are several types of keys each having slightly different characteristics:

    Super key-is defined as a set of attributes within a table that can uniquely identify
    each record within a table. Super Key is a superset of Candidate key.

    Candidate key – A candidate key is an attribute or set of attributes that uniquely
    identifies a record in a relation. A candidate key is unique and minimal.

    It is unique because no two rows in a table may have the same value at any time. It is
    minimal because every column is necessary in order to attain uniqueness.

    From our Company database example, if the entity is Employee(SerialNo,EmpId,
    First Name, Surname, Address, Contact, Birthdate, Salary, Depid), possible candidate
    keys are:


    • SerialNo

    FirstName and Surname – assuming there is no one else in the company with
    the same full name

    Surname and Depid – assuming two people with the same Surname don’t
    work in the same


    Composite keys – these keys have multiple attributes.
    Using the example from the candidate key section, possible composite keys are:

    • First Name and Surname

    • Surname and Depid

    Primary key – A primary key is one of the candidate keys from a relation. Every
    relation must have a primary key. A primary key shall be at least: - Stable. The value
    of a primary key must not change or become null throughout the life of the entity.

    The primary key is indicated in the ER model by underlining the attribute.

    For example, consider an employee record; using the Age field as the primary key
    would not be appropriate because the value of the primary key must not change
    over time.

    It should be Minimal. The primary key should be composed of the minimum number
    of fields to ensure the occurrences are unique.

    In this case Employee (EmpId, First Name, Surname, Address, Contact, Birthdate,
    Salary, Depid), EmpId is primary key

    Secondary key

    A secondary key is an attribute used strictly for retrieval purposes (can be composite),
    for example: Phone and Surname.
    Alternate key – An alternate key is any candidate key that is not chosen to be the

    primary key. It may become the primary key if the selected primary key is not

    Simple keys – these keys have a single attribute.

    Foreignkeys – these keys exist usually when there are two or more relations. An attribute
    from one relation has to exist in the other(s) relation. A foreign key is a field (or
    fields) that points to the primary key of another table. The purpose of the foreign key
    is to ensure referential integrity of the data. In other words, only values that are supposed
    to appear in the database are permitted
    In the Company database example below, Depid is the foreign key
    (EmpId, First Name, Surname, Address, Contact, Birthdate, Salary, Depid)

    Attributes and Keys

    • Key attributes must be unique for each entity

    • Keys are used to identify particular entities

    • Partial keys are only partially unique used for weak entity types

    Entity Types and Attributes

    All regular entity types must have a key attribute or set of key attributes, Weak entity
    types must have partial keys, Weak entities get part of their key (and part of their
    identity) from some related entity.


    A null is a special symbol, independent of data type, which means either unknown
    or inapplicable. It does not mean zero or blank.

    Features of null include:

    • No data entry

    • Not permitted in the primary key

    • Should be avoided in other attributes

    • It can represent

    an unknown attribute value

    1. a known, but missing, attribute value

    2. a “not applicable” condition

    Example of null

    Employee Salary table

    In this case commission can be left null

    Relationship sets

    A relationship set is a set of relationships between two or more sets of entities, and
    are regularly represented using a verb.

    A relationship is an instance of a relationship set and establishes an association
    between entities that are related. These relationships represent something important
    in the model. Example: an employee works on a project.

    A relationship set always exists between two entity sets (or one entity set relating
    to itself ). You need to read a relationship set in double sense, from one entity set to
    the other.

    Relationships representation

    Relationships => diamonds
    Identifying relationship => double diamond

    Relationships indicate a meaningful connection between two entity types,
    Relationships may have attributes, but they cannot have key attributes. Identifying
    relationships connect a weak entity type to some other entity type indicates where
    the weak entity gets a key to complete its own partial key

    5.4.7. Degree of a relationship

    The degree of a relationship refers to the number of associated entities. The degree of
    a relationship can broadly be classified into unary, binary, and ternary relationship. Unary Relationship

    The unary relationship is known as recursive relationship.
    In the unary relationship the number of associated entities is one. An entity is related
    to itself is known as recursive relation.
    An example is shown by the following figure:
    Roles and Recursive Relations

    When an entity set appears in more than one relationship, it is useful to add labels to
    the connecting lines. These labels are called roles.
    In this example, labels “Husband” and “wife” are referred to as roles. Binary Relationship

    In a binary relationship, two entities are involved. Consider the example where each
    staff will be assigned to a particular department. Here the two entities are STAFF and
    DEPARTMENT. Ternary Relationship

    In a ternary relationship, three entities are simultaneously involved. Ternary
    relationships are required when binary relationships are not sufficient to accurately
    describe the semantics of an association among three entities.


    Consider the example of employee assigned a project.
    Here we are considering three entities EMPLOYEE, PROJECT, and LOCATION. The
    relationship is “assigned-to.” Many employees will be assigned to one project hence
    it is an example of one-to-many relationship. Quaternary Relationships

    Quaternary relationships involve four entities. The example of quaternary relationship
    is “A professor teaches a course to students using slides.” Here the four entities are
    The relationships between the entities are “Teaches.

    The degree of relationship, which determines how many instances of an entity relate
    to a single instance of another entity is called Cardinality.

    5.4.8. Classifications or types of Relationships

    Based on cardinality, the different types of relationships that can exist between
    entities are: One-to one (1:1)

    A single instance of an entity can relate to only one instance of the other entity.It
    is the relationship of one entity to only one other entity, and vice versa. It should
    be rare in any relational database design. In fact, it could indicate that two entities
    actually belong in the same table. An example from our Company database is one
    employee is associated with one spouse, and one spouse is associated with one

    Other Examples:

    • A person can have only one passport.

    • The relationship between the President and the country is an example of oneto-
       one relationship. For a particular country at a given time, there will be only
       one President. In general, a country will not have more than one President at
       a given time hence the relationship between the country and the President is
       an example of one-to-one relationship. One-to-many (1: M)

    An instance of one entity can relate to multiple instance of another instance.

    The relationship that associates one record of entity A to more than one record of
    entity B is called one-to-many relationship.

    Example of one-to-many relationship is a country having states.

    For one country there can be more than one state hence it is an example of oneto-
    many relationship. Another example of one-to-many relationship is parent–child
    relationship. For one parent there can be more than one child. Hence it is an example
    of one-to-many relationship.

    For Example,

    • A class has many students.

    • The relationship between EMPLOYEE and DEPARTMENT is an example of
    many-to-one relationship. There may be many EMPLOYEES working in one
    DEPARTMENT. Hence relationship between EMPLOYEE and DEPARTMENT is
    many-to-one relationship. Many-to-One Relationship Type (M: 1)

    The relationship between EMPLOYEE and DEPARTMENT is an example of many-to-one
    relationship. There may be many EMPLOYEES working in one DEPARTMENT. Hence
    relationship between EMPLOYEE and DEPARTMENT is many-to-one relationship Many-to-many (M: M):

    Multiple instances of an entity can relate to multiple instances of another entity.
    For our company database. The relationship between employee and department
    is many-to-many relationship.

    Other examples are:

    • A customer can purchase more than one book.

    • The relationship between TEACHER entity and STUDENT entity is an example
      of many-to-many relationship. Many one teacher teaches many students and
      one student is taught by many teachers.
      Employee and project, many employees can work on many projects

    Participation and Cardinality

    Participation and cardinality define constraints on relationships; Participation
    indicates whether an entity is required to take part in a relationship, Cardinality ratios
    and structural constraints place limits on the number of entities that may participate
    in a relationship.

    Participation Constraints

    • Total participation → double or thick line indicates required participation

    • Partial participation →thin line indicates optional participation

    Participation Constraint

    Arrowheads can be used to indicate an upper bound of 1 for participation

    Cardinality Ratios: a cardinality ratios specify the maximum number of relationship
    instances that an entity may participate in.

    Cardinality Ratios: a cardinality ratios specify the maximum number of relationship
    instances that an entity may participate in.

    Structural Constraints

    Structural constraints specify the minimum and maximum number of relationship
    instances that an entity may participate in.

    An employee must work for exactly 1 department. A department must have at least
    4 employees.

    Participation and Cardinality

    There’s generally numerous ways to express a relationship constraint.

    Equivalent Notations

    An employee can manage at most one department.
    A department must have exactly one manager.

    Equivalent Notations

    • An employee must work for exactly one department.

    • A department must have at least one employee.

    Application Activity 5.5

    Given an ER diagrams below

    Write the degree, cardinality, participation and Constraints of each relationship in
    ER diagram

    Write the names of all: weak entities, composite attributes, multivalued attributes,
    composite keys and Partial Key Attribute, derivative attribute, identity relationship.

    Relationships representation

    1. Consider a database used to record information about the marks that
    students get in the different exams of different subject studying in Math-
    Computer Science-Economics combination.

    STUDENT entity is described by StudentId, StudentName and School; SUBJECT_
    STUDYING is identified by SSId, SSName, SSClass and SSRoom and EXAM entity is
    described by EId, EName, ETime and ERoom.

    Construct an E-R diagram that models STUDENT, SUBJECT_STUDYING and EXAM
    as entities that uses ternary relationship.

    5.5. Database optimization through normalization

    Learning Activity 5.6.

    Let consider the database with table CUSTOMER identified by CustomerName,
    Address, TelephoneNumber, CreditLimit, ItemOrdered,Quantity and Price.
    This can easily be implemented in a relational database as follow:

    However, a customer may order several items and each customer in the
    database may order a different number of items. This situation makes it difficult
    to implement the data in a relational database, since we do not know how many
    order entries to allow.

    1. How many order entries our database design would allow every

    2. Is this structure simple?

    3. If this database structure is complex, identify the anomalies it has and suggest
        the solution to the database designer to make it simple.

    5.5.1. Introduction

    Another method for designing a relational database is to use a process commonly
    known as normalisation. The goal is to generate a set of relation schemas that
    allows us to store information without unnecessary redundancy, yet also allows
    us to retrieve information easily. The approach is to design schemas that are in an
    appropriate normal form. Therefore, normalisation is a part of the database design
    process. Normalisation is a series of steps designed to remove repeating groups
    and unwanted functional dependencies. Normalisation is a design technique for
    constructing a set of table designs from a list of data items. . It is also used to avoid
    insertion, deletion and updating anomalies. Therefore it used to improve on existing
    table designs.

    5.5.2. Normal Forms

    Normalization theory defines six normal forms (NF). Each normal form involves a set
    of dependency properties that a schema must satisfy and each normal form gives
    guarantees about the presence and/or absence of update anomalies. To correct
    update anomalies in a database, you must convert tables to various types of normal

    The most common normal forms are First Normal Form (1NF), Second Normal Form
    (2NF), Third Normal Form(3NF) and Fourth Normal Form(4NF).

    a. First Normal Form

    A relation (table) that contains a repeating group (multiple entries for a single record)
    is called unnormalised relation. Removing repeating groups is the starting point in
    the quest to create tables that are as free of problems as possible. The conversion
    to first normal form (lNF) requires splitting the data into two groups. Tables without
    repeating group are said to 1NF.

    A relation is in first normal form if and only if the domain of each attribute contains
    only atomic (indivisible) values, and the value of each attribute contains only a single
    value from that domain


    A database has been designed to store data about sellers and the products they
    have sold.
    The following facts help to define the structure of the database:

    • each seller works in a particular shop

    • each seller has a unique Surname

    • each shop has one or more sellers

    • each product which is sold is manufactured by one company only

    • each seller can sell any of the products

    • the number of products that each seller has sold is recorded

    This table is not in 1NF because:

    1. SHOPSALES table has repeated group of attributes

    (ProductName, Manufacturer)

    (ProductName, NumberofProducts),

    (NumberofProducts, Manufacturer)

    2. each seller has a number of products

    3. SurName and ShopName would need to be repeated for each record

    The following table is in 1NF because it does not contain repeating groups

    b. Second Normal Form (2NF)

    For the second normal form, the relation must first be in 1NF. The relation is
    automatically in 2NF if, and only if, the Primary Key comprises a single attribute.

    To move to 2NF, a table must first be in 1NF.

    The database is changed to the following design:

    SELLER (SurName, ShopName)

    PRODUCTSSOLD (SurName, ProductName, NumberofProducts, Manufacturer)

    Table SELLER is in Second Normal Form because attribute ShopName depends on
    primary key SurName; the same on PRODUCTSSOLD where NumberofProducts
    and Manufacture attributes depends on composite primary key SurName,
    ProductName. The link between these two tables is: primary key of SELLER table is
    FirstName, links to FirstName in PRODUCTSSOLD table, FirstName in SalesProducts
    table is foreign key

    c. Third Normal Form (3NF)

    To be in third normal form, the relation must be in second normal form. Also
    all transitive dependencies must be removed; a non-key attribute may not be
    functionally dependent on another non-key attribute.

    In our case on table PRODUCTSSOLD, Manufacturer attribute is dependent on
    ProductName, which is not the primary key of the PRODUCTSSOLD table therefore
    there is no key dependency.

    Process for 3NF:

    1. Eliminate all dependent attributes in transitive relationship(s) from each of
        the tables that have transitive relationship.

    2. Create new table(s) with removed dependency.

    3. Check new table(s) and modified table(s) to make sure that each table
       does not contain contains inappropriate dependencies.

    See the three new tables below.

    SELLERS (SurName, Shop)

    PRODUCTSSOLD (SurName, ProductName, NumberofProducts)

    PRODUCTS (ProductName, Manufacturer)


    d. Boyce-Codd Normal Form (BCNF)

    When a table has more than one candidate key, anomalies may result even though
    the relation is in 3NF. Boyce-Codd normal form is a special case of 3NF. A relation
    is in BCNF if, and only if, every determinant is a candidate key.


    Consider the following table STUDENT
    The following facts help to define the structure of the database:

    1. Each Student may major in several Combinations.

    2. For each Combination, a given Student has only one ClassTeacher.

    3. Each Combination has several Advisors.

    4. Each ClassTeacher advises only one Combination.

    5. Each ClassTeacher advises several Students in one Combination.

    The functional dependencies for this table are:

    1. StudentId, Combination→ClassTeacher

    2. ClassTeacher →Combination

    Anomalies for this table include:

    • Delete→Student deletes ClassTeacher inforamtion

    • Insert→a new ClassTeacher needs a Student

    • Update →inconsistencies or contradiction

    Because no single attribute which is a candidate key, primary key can be

    StudentId, Combination, Student_id,ClassTeacher.

    To reduce STUDENT table to BCNF, there is creation of two tables:

    • STUDENTTEACHER (Studentid, ClassTeacher)

    • TEACHERCOMBINATION(ClassTeacher, Combinationr)

    e. Fourth Normal Form (4NF)

    Fourth normal form eliminates independent many to one relationships between
    A relation is in Fourth Normal Form:

    • A relation must first be in Boyce-Codd Normal Form.
    • Given relation may not contain more than one multivalued attribute.

    The multi valued dependency X→Y holds in a relation R if whenever there is two
    tuples of R that same in all the attributes of X, then we can swap their Y components
    and get two new tuples that are also in R.

    Primary key→{StudentId , Subject , Clubs}

    • Many StudentId have same Subject.

    • Many StudentId have same Clubs.

    Thus violates 4NF.
    To convert to 4NF, the table STUDENTCLUB is changed to the following design:
    STUDENTSUBJECT {StudentId, Subject}

    Is this normalised table? If not,
    convert it to 1NF, 2NF and 3NF.

    2. Consider the following table SUBJECT that contains multivalued
        dependency thus it is in 3NF, use it to answer asked questions

    i. Identify functional dependencies exist in this tabale above

    ii. Eliminate those dependencies.


    Question 1.

    The database below is composed of two tables. Use it to answer asked questions

    i. What is primary key for each table and give reasons.

    ii. Identify the foreign key in the BOOKS table.

    iii. Identify the candidate keys in both tables.

    iv. Identify the relationship between BORROWERS and BOOKS tables
        and draw the E-R model.

    v. Does the BOOKS table exhibit referential integrity? Why or why not?

    Question 2.

    Write an ER diagram for Banking System. Assume your own entities (minimum 5),
    attributes and relation. Mention the cardinality ratio.

    Question 3.

    A picture gallery owner has decided to set up a database to keep information
    about the pictures he has for sale. The database table, PICTURE, will contain the
    following fields:

    PictureTitle; PictureArtist; PictureDescription; CatalogueNumber; PictureSize (in
    MB); PicturePrice; ArrivedDate (date picture arrived at gallery); Status(whether
    picture is already sold)

    PictureTitle; PictureArtist; PictureDescription; CatalogueNumber; PictureSize (in MB);
    PicturePrice; ArrivedDate (date picture arrived at gallery); Status(whether picture is
    already sold)

    i. State what data type you would choose for each field.

    ii. State which field you would choose for the primary key.

    iii. Complete the query-by-example grid below to select and show the

    CatalogueNumber, PictureTitle and PicturePrice of all unsold pictures
     by the artist ‘GATOTO’.