Databases and Database Management Systems (DBMS)
A Database
There are many definitions of a database
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An organised collection of related data
An organised collection of files in a specific subject area
A filing cabinet is a database. An address book or telephone directory is a database. However, we
are concerned with computerised databases, that is files that are stored in an organised way in a
computer using a database software application
Database Management System (DBMS)
Special software for managing a database which allows users to record, store, query, change and
update the data stored in the database. It also allows the data to be manipulated in many ways
using queries, forms, reports etc
Database Models
The database model is to do with how the data is organised and the relationships between different
data items in the database. This is transparent to the user but important for the person who
organises the database. Access is a relational database model. This means that it stores data in
tables (or relations as they are correctly called!). Relationships are made between tables and in
this way we can access data that is stored in a number of tables in a single query. Other database
models are the Network model, the Hierarchical Model and the Object-oriented model. We are
only concerned with the relational model
Data Structures
We record data about objects that we see around us in the real world. An object might be a
person, place or in fact anything that we can describe. The object we describe is called an
'Entity'.
Fields are used to describe the entity ie for the entity Customer we might have the customer
address, name, telephone no etc and these will all be stored in fields. These items of information
are 'attributes' that describe our entity
Data is organised according to fields, records, files
Fields hold the attributes
Each field in the same table must contain the same type of data
A collection of fields about a certain object is called a record
A record stores all the attributes on a single entity (Customer)
A group of related records is called a file so in our relational database each of the tables represents
a 'file'
A Database
This is a collection of files within an organisation
Flat File Organisation
Until the advent of DBMS most organisations previously used a flat file system (until the 1970's). A
flat file system meant each section of the organisation maintained its own filing system. There was
no relationship between the methods used in different department or units in an organisation. It
meant that often the same data was recorded in different departments leading to anomalies and
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waste of resources. It was often difficult to access information and there were many different
programs used so often the data could not be linked
Result
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DUPLICATION of data - data redundancy
RESOURCE WASTAGE - computer and human as the same data was stored and maintained in a
number of different places in the firm
ACCESS TO INFORMATION - difficult as it was spread all over the organisation
DIFFERENT STANDARDS within the same company. The same customer name might be
recorded differently in 2 different files thus giving the impression that there were 2 customers
instead of just one
NO RELATIONSHIP between different databases in the organisation. This integration of data
difficult
Increased opportunity for error. If two files must be updated instead of one there is a greater
chance for error
DBMS
A DBMS solves many of the problems mentioned above. Data is recorded only once in the database
thus it only needs to be updated in a single location
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Easier access to the data in the database
Through use of relationships there is greater data integrity
Data can be shared easily between different departments
Better data security as it is held in one place
Setting up a Database
The steps in creation of a database are
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Designing the structure
Inputting the data
Database Terminology
Entity
This is the object, person, place or thing on which we wish to record data eg Student, Grant,
Course
Attribute
This is something that describes an entity. If the entity is a student then some of the student's
attributes would be Name, Address, Student No, Tel No, Course, etc
Field
This is the location in the database in which we record the attributes. The database designer
decides on the fields when the database is being constructed initially. All fields with the same
name should contain the same time of data. A field can contain numeric, alphabetic or logical data
to name but some. A field is just a location in which the database holds data.
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Record
A record contains all the data we have recorded on a certain 'entity'. Each Row in a table
represents a Record
Structure Design
Involves describing what data is needed to be recorded in our database
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Decide on field names
Determine field type which can be any of the following
(i) Text
(ii) Number
(iii) Data
(iv) Logical
(v) Memo
(vi) Date
Determine field width
Define key fields (The Primary Key uniquely identifies a record)
The desired structure is set up using the software (MsAccess etc)
Once the database structure has been decided and set up, data can be input
Features of a DBMS
The DBMS makes it easy for the user to carry out the following tasks
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List records (all or selected)
List only certain fields
Query - request data from the database
Create reports
Sort records
Update records
Add/Delete records
Advantages of DBMS
The DBMS is the user interface. It also has the following advantages over other methods of storing
data
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Reduced data redundancy
Easy retrieval of data
Easy update of data
Integrated data - so reports can be generated easily
Data integrity
Allows connectivity - over a network
Allows different views
Data security easier to implement
A DBMS allows the whole organisation to use a company-wide database. This is called a
Management Information System. Information is available to the members of the organisation
where and when they need it.
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Database Models
Not all DBMS use the same methods to organise and store data. Different models used are:
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Hierarchical
Network
Object-oriented
Relational
The models differ in how they organise the data. This is often transparent to the user
Relational Database Model
The logical view of the relational model is as follows:
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Tables (Relations) with rows and columns. A column represents a field and a row represents a
record
The complete set of tables (relations) is a file
A number of these files make up the company database
The Relational Model Structure
Let us take a small example of the College Database. This is an oversimplification of the college
database but it should do to illustrate the concepts. Here we see 3 files - the student file, the
Course file and the Grants file.
Each file is shown as a Table
Tables in a relational database are called 'relations'
Links can be made between tables by linking fields that contain the same type of data
Above is an extract of what the college Student File might look like.
Field Contents or 'Attributes'
Field Names eg Grant_Status and Student_ID, Street, Town etc
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Here is what the Grants file might look like. We record whether or not each student is in receipt of
a grant
Below we have the Course table that shows what each course code means
These are just 3 of the many possible files that will make up the college database. Realistically it
could have any number of files each representing an entity on which we have to maintain records
Advantages of Relational Database Model
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Ease of update and retrieval of data
More information from same data (we can join tables to perform a query)
Data sharing is made easier (distributed databases)
Enforcement of standards
Controlled data redundancy (sometimes we need to repeat a field in another table so that we
can create a relationship between the tables. This is OK)
Consistency of data (use of input masks and data formats to ensure consistency)
Data Integrity (enforcing integrity rules)
Security (data in one location easier to secure)
Disadvantages
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Size
Complexity
Cost
Backup procedures (Since data only in one place it would be critical if data is lost. Good
backup procedures necessary)
Additional hardware and software requirements
Higher impact of failure
Recovery more difficult
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