Download Unit 5.2 – Database Management System (DBMS)

GCSE Computing A451
Unit 5.2 – DBMS
www.mrfraser.org
Unit 5.2 – Database Management System (DBMS)
Candidates should be able to:
a. Describe how a DBMS allows the separation of data from applications and why this is
desirable.
b. Describe the principal features of a DBMS and how they can be used to create customised
data handling applications.
The DBMS
The database management system (DBMS) is a software application that sits between the
data in the database and the applications that an organisation uses to access that data.
Its key roles are:
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Control creation, maintenance and use of a database
Control access to data
Maintain data security
Enforce data integrity
DB recovery after failure
Database Integrity
Database integrity ensures that data entered into the database is accurate, valid, and
consistent. Any applicable integrity constraints and data validation rules must be satisfied
before permitting a change to the database. Integrity can be compromised if unintended
changes take place by accident or by malicious individuals.
Three basic types of database integrity constraints are:
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Entity integrity – Each row in a table must have a unique identifier
Domain integrity – Data must be of a predefined type (e.g. String, date, integer)
Referential integrity – Every value of one attribute (column) of a table must exist as a
value of another attribute in another table.
Some changes to a database might not make sense. For example, if you delete songs
from your music library, but they are still referred to in a playlist... the playlist will not be
able to locate the songs to play them. Or, in a school, there might be information about
which pupils are taking which exams. If someone deleted an exam subject and there
were pupils entered for it, this would make it impossible to do the entry processing.
DBMSs allow the database designer to enforce referential integrity which prevents
inconsistent changes from being carried out.
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GCSE Computing A451
Unit 5.2 – DBMS
www.mrfraser.org
Database Security
There is a wide range of security controls employed to protect databases against
compromises of their confidentiality, integrity and availability.
Security risks to database systems include:
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Unauthorised access to database
Unintended activity or misuse by authorised database users
Malware infections – e.g. causing leakage or disclosure of personal data
Overloads – Database cannot be used due to performance constraints & capacity issues
Physical damage – Fire, flood, overheating, lightning, accidental liquid spills etc.
Design flaws and programming bugs in databases – resulting in security vulnerabilities
Data corruption
Data Access
In order to ensure data integrity, only one copy of the data is held in a database. It is
necessary, therefore, for the DBMS to control access to specified data within the database
at any given time. Multiple, different applications must be able to access the same database
simultaneously, but not edit the same data at the same time (whilst one user/application
edits a record, other users/applications will be restricted to read-only access to that record.)
The DBMS is also responsible for maintaining the different external views of the database
available to different users (see notes from Unit 5.1 – Database Concept). Different people
need different subsets to work on. They must all, however, be able to see the one and only
up-to-date copy of the data, so that there are no inconsistencies.
The DBMS must:
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Allow data sharing
Allow multiple user access
Allow multiple applications to work on the same database
Hold a single copy of data for all to access to prevent inconsistent updates
Allow data to be changed without having to change the applications using it
Provide multiple views of the same data.
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GCSE Computing A451
Unit 5.2 – DBMS
www.mrfraser.org
ACID
There is a set of rules that databases should always be subjected to so that inconsistent
changes are not made. These are known as the ACID rules. They refer to transactions
(actions that bring about a change to the data).
Atomic – a change is either completely performed or not at all.
It must not be possible to abandon a transaction and save it in a
half completed state. For example, in a bank, if one account is
debited, another must be credited or the money just
disappears.
Consistent – a transaction must take a database from one
consistent state to another. Again, in a banking situation, the
total amount of debits in the complete system must equal the
total credits.
Isolated – a transaction should not be visible to any other transaction or user until it is
irrevocably committed. Imagine if several theatre booking offices had access to a particular
seat in a particular theatre on a particular day, all at the same time: double bookings could
easily occur. It is necessary to lock a record while a transaction is taking place and not
release it until it has been committed.
Durable – once a change has been made, it must not be lost due to any subsequent failure
Applications
DBMSs allow the creation of applications. The DBMS makes sure that any applications
related are not able to damage the underlying data. Applications can be created by the
language provided or often by using a visual interface. This allows the designer to create
objects such as:
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Tables
Forms
Queries
Reports
Modules
It is often possible to create the database objects using a wizard. This asks questions about
what is required and then constructs the component automatically.
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GCSE Computing A451
Unit 5.2 – DBMS
www.mrfraser.org
Tables
These are the essential data stores of the database. The fields can be created manually or a
wizard can suggest fields for you. It is important to make the right decisions about which
data goes into which table. Also, data types have to be chosen so that they can help in the
validation of data input and ensure the database performs in the way intended.
Forms
These are the interface between the user and the underlying application. They can accept
data as well as display it. They can have controls added, such as buttons and combo boxes,
in order to provide interaction and validation capability.
Queries
These are used to extract a subset of the data. They can be applied to more than one table
in order to provide the user with exactly the right data to work on. Queries can be
constructed visually as in the screenshot on the next page. The tables and fields are inserted
where required. The conditions can be added by filling in boxes. Alternatively, SQL can be
used to construct a query (see notes from Unit 5.1 – Database Concepts).
Reports
These are printed lists of selected data from a database. They can be set to present data in
all sorts of ways. They can be based on tables or queries in order to select exactly the data
that is needed.
Modules
These are units of program code. Most DBMSs have a programming language built in so that
a developer can have total control over the performance of the applications. For example,
Microsoft® Access has the language VBA (Visual Basic® for Applications) built into it. Here is
a short example of VBA code.
Sub RunMyList()
Dim MyDB As Database, MyRecs As Recordset, MyName As String
Set MyDB = CurrentDb()
Set MyRecs = MyDB.OpenRecordset("MyList")
MyName = InputBox("Enter your name")
MyRecs.MoveFirst
End Sub
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