Database Design Concepts Info1408 Week 5 Database Components

Objectives Two basic concepts to explore: Conceptual Design Components Logical/Physical Design Components

Technology independent- ie will work on any database software Conceptual Design Technology independent- ie will work on any database software An activity carried out by a

Logical Design Process of translating (mapping) the conceptual design to fit the chosen database model (relational, object oriented etc.) An activity carried out by a

Physical Design Mapping the logical design to one or more physical designs Tailored to the particular DBMS being used and the computer system on which the database will run. An activity usually performed by a

Entities Something of interest to the activity we are trying to model Could be a person, place, thing, concept about which data is collected Just about anything that can be named with a noun can be an entity …we should restrict ourselves to things of interest to the users of our database.

A very common entity that is found in many business oriented databases is the Conceptually this represents all the customers in the database ……… an individual customer is known as an of the entity CUSTOMER

Attributes ‘An attribute is a unit fact that characterises or describes an entity in some way’ In other words properties Attributes can not be broken down into smaller units in any meaningful way … they are the smallest unit of information that is stored within the database Examples could be name, address, telephone number of the customer

Unique identifiers An entity will be described using the attributes. For example a student could have name and results. The problem is we need to be identify one student from another. Most students can be identified form one another by their name however sometimes two students can have the same name. How do I make sure I don’t award marks to the wrong student?

I do this by adding an extra attribute known as the key field. This is a unique identifier Each student will have a different value for this attribute ie it is unique What is your unique identifier ? Remember these unique identifiers are often invented so are often not a property the subject would recognise.

Examples of unique identifiers National insurance number Passport number Membership number Order number Vehicle registration number Bank account number Remember that only one instance of the entity will have each individual value ie it is unique.

An entity is composed of an example: the customer entity can be represented as …. CUSTOMER_ID COMPANY_NAME ADDRESS CITY COUNTRY PHONE Unique identifier attributes

Relationships Relationships are the among entities Relationships are represented at the conceptual level by lines joining entities together. Each end of the relationship line shows the maximum cardinality of the relationship (maximum number of instances of one entity that can be associated with the entity at the other end of the relationship line)

Entity-Relationship modelling It is important to realise that there are a number of different ways of representing the details of relationships and cardinality For the purposes of this module we want to explore the underpinning concepts involved … we will use one notational style …. You will come across different styles as you refer to different text books. We will cover this later in the course

Business Rules Very important …… a policy, procedure or standard that an organisation has adopted. These business practices must be reflected in the construction and functioning of the database They may impose on the database design

Business Rules It is important to identify business rules during investigation of the data requirements. Business rules will vary from organisation to organisation. It is important to identify the business rules for each relationship.

Examples of Business Rules A student can be enrolled on one course only- may be appropriate at a university A student can be enrolled on more than one course – May be appropriate in a college. A customer can place many orders. An order is for one customer

Logical Physical (1) Tables Primary unit of storage in the relational model is the table A two dimensional structure composed of rows and columns Each row represents one occurrence (instance) of the entity ( or record) Each column represents one attribute for that entity.

For each attribute you must identify it’s properties. Type- this can be TEXT NUMBER DATE/TIME CURENCY Size- how big Is it mandatory or optional

Summary You should be able to define the following terms Entities Attributes Relationships Business rules Tables Columns Data types Constraints

References Database Demystified by Andy Oppel