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8/4/2019 Hodder Advanced Databases
1/21
A Level Computing for AQA Teachers Resource CD-ROM
41CHAPTER:
Advanced database
concepts
8/4/2019 Hodder Advanced Databases
2/21
A Level Computing for AQA Teachers Resource CD-ROM
2005 Bob Reeves, Dave Fogg/Hodder Murray
Syllabus
Explain the purpose of a database management system
(DBMS), query languages and data dictionaries.
Describe the advantages of different users having different
views of the data in a database.
Discuss different approaches to database security. Recognisethat the individual user of a database may be prevented from
accessing particular elements of the information.
Explain the role of the database administrator.
Explain what is meant by data warehousing and data mining,
using examples from supermarkets and insurance companies.
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A Level Computing for AQA Teachers Resource CD-ROM
2005 Bob Reeves, Dave Fogg/Hodder Murray
SQL
SQL is a language used to extract data from relational
databases.
It is available in various versions and embedded into database
applications, such as Microsoft Access.
It performs the same function as a QBE, in that it searches andsorts data.
When you create a QBE, Access writes the SQL automatically.
SQL is written in lines of code (like a programming language) to
extract and sort data from several entities within a relational
database.
8/4/2019 Hodder Advanced Databases
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A Level Computing for AQA Teachers Resource CD-ROM
2005 Bob Reeves, Dave Fogg/Hodder Murray
SQL statements
A typical SQL statement might look like this:
SELECT CustomerName, Address
FROM Customer
WHERE CustomerName = John SmithORDER BY CustomerName DESC
This statement will extract the customer name and address from
the Customerentity of all customers called John Smith sorting
the results in descending order based on the customer name.
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A Level Computing for AQA Teachers Resource CD-ROM
2005 Bob Reeves, Dave Fogg/Hodder Murray
SQL commands
SELECT identifies the attributes that you want to extract. These
can be from one or more tables.
FROMindicates the table or tables that the data is to be
extracted from.
WHERE is the condition that must be met. These can be complex(using AND/OR) or you may not have any conditions at all.
ORDER BY indicates the sort order of the extracted data. The
default is ascending so DESC is added to make it descending.
8/4/2019 Hodder Advanced Databases
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A Level Computing for AQA Teachers Resource CD-ROM
2005 Bob Reeves, Dave Fogg/Hodder Murray
Further examples
SELECT *
FROM Customer
WHERE CustomerName = "John Smith" or "Mary Jones"
will extract all attributes (because the wildcard * is used) fromthe customer entity where the name is either John Smith or
Mary Brown.
SELECT *
FROM VIDEO
WHERE VideoPrice BETWEEN 2 AND 2.50
will extract all attributes from the video table where the price of
the video is between 2 and 2.50.
8/4/2019 Hodder Advanced Databases
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A Level Computing for AQA Teachers Resource CD-ROM
2005 Bob Reeves, Dave Fogg/Hodder Murray
Querying more than one entity
A more useful query would be to extract the names and
addresses of all customers with an overdue video, along with
the name of the video and the date it was taken out.
These customers can then be contacted to remind them to bring
it back. For example, the extracted data could be used togenerate a report or mail merged with a reminder letter.
This process will involve querying all three entities.
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8/4/2019 Hodder Advanced Databases
9/21
A Level Computing for AQA Teachers Resource CD-ROM
2005 Bob Reeves, Dave Fogg/Hodder Murray
The data to be queried: VIDEO
VideoID VideoName Genre AgeClassification Price
1000 Titanic Drama 12 2.00
1001 Matrix Sci-fi 15 2.50
1002 Training Day Action 18 3.00
1003 Star Wars Sci-fi 12 2.00
1004 Pearl Harbour Action 15 2.50
8/4/2019 Hodder Advanced Databases
10/21
A Level Computing for AQA Teachers Resource CD-ROM
2005 Bob Reeves, Dave Fogg/Hodder Murray
The data to be queried: RENTAL
RentalID DateHired CustomerID VideoHired OverdueYN
0001 19/03/03 2000 1000 Y
0002 19/03/03 2000 1001 Y
0003 19/03/03 2000 1001 N
0004 19/03/03 2001 1002 Y
0005 19/03/03 2001 1003 N
0006 19/03/03 2002 1004 N
8/4/2019 Hodder Advanced Databases
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8/4/2019 Hodder Advanced Databases
12/21
A Level Computing for AQA Teachers Resource CD-ROM
2005 Bob Reeves, Dave Fogg/Hodder Murray
Results of the query
The query would extract the following data:
CustomerName Address DateHired VideoID VideoName Price
John Smith 1 High Street 19/03/03 1000 Titanic 2.00
John Smith 1 High Street 19/03/03 1001 Matrix 2.50Mary Jones 14 Acacia Avenue 19/03/03 1002 Training Day 3.00
Now work through the SQL question sheet
8/4/2019 Hodder Advanced Databases
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A Level Computing for AQA Teachers Resource CD-ROM
2005 Bob Reeves, Dave Fogg/Hodder Murray
Database management system (DBMS)
A DBMS is a program that controls:
the data that is kept on the database
how it is stored
where it is stored
who has access to it.
Many databases (particularly in large organisations) are
accessed by many different users for different purposes.
This can cause problems as different users may alter the data.
Different users may also need different programs to manipulatethe data.
If the data changes they have to adjust their programs, which is
known as unproductive maintenance.
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A Level Computing for AQA Teachers Resource CD-ROM
2005 Bob Reeves, Dave Fogg/Hodder Murray
The DBMS layer
THE DBMS acts as a layer of
software between all the users
and the database.
The following diagram shows
the concept for a typical
business.
All the departments can access
the data without having direct
access to the database.
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A Level Computing for AQA Teachers Resource CD-ROM
2005 Bob Reeves, Dave Fogg/Hodder Murray
Data dictionary
The DBMS records the structure of the database.
It records the attributes, data types, validation used and the
relationship between entities.
Any changes to the data structure are recorded in the data
dictionary.
Any requests to access the data are directed via the data
dictionary. The DBMS then presents the appropriate table.
The data dictionary does not store the data itself. It records the
structure of the data.
This leads to the concept ofprogram-data independence. Thedata and the programs that are used to access it are not directly
linked.
8/4/2019 Hodder Advanced Databases
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A Level Computing for AQA Teachers Resource CD-ROM
2005 Bob Reeves, Dave Fogg/Hodder Murray
Controlling the data
The DBMS controls what data each user is allowed to see.
For example:
The finance department may have access to details on
wages, whereas the training department may have access to
employment history.
It also controls what they are allowed to do with the data.
For example:
Some departments may be able to see the data, while others
will have access rights to amend it.
8/4/2019 Hodder Advanced Databases
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A Level Computing for AQA Teachers Resource CD-ROM
2005 Bob Reeves, Dave Fogg/Hodder Murray
Database schema
There are three views of the database:
external schema: the way in which the users see the database.
The complexities of the data are hidden from the user so this
view is the front-end
conceptual or logical schema: describes the structurerelationships and the entities and attributes
internal schema: describes how the data will be stored
physically and how it will be accessed and updated.
8/4/2019 Hodder Advanced Databases
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A Level Computing for AQA Teachers Resource CD-ROM
2005 Bob Reeves, Dave Fogg/Hodder Murray
Data integrity on a shared database
Where users are sharing data there is a danger that they could
attempt to alter data at the same time.
Multiple users can view data simultaneously but not edit it.
The DBMS could give access rights to the first user who opens
a record and read-only rights to any subsequent user.
This means that the record is locked until the first user has
finished editing it.
8/4/2019 Hodder Advanced Databases
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A Level Computing for AQA Teachers Resource CD-ROM
2005 Bob Reeves, Dave Fogg/Hodder Murray
Database servers
In a network, a dedicated server may hold the database.
Access to the database therefore is via this server and all
changes made to the database are made on the copy stored on
the server.
On a traditional network, the server would store the databaseand send it to the workstation when a user requested it.
The integrity of the data is maintained with a database server as
there is only ever one copy.
8/4/2019 Hodder Advanced Databases
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A Level Computing for AQA Teachers Resource CD-ROM
2005 Bob Reeves, Dave Fogg/Hodder Murray
Database administrator
The database administratoris the person responsible for
maintaining the database, including...
designing the database
keeping users informed of changes
maintenance of the data dictionary for the database
implementing database security measures
allocating passwords
providing training to users
ensuring adequate backup procedures.
8/4/2019 Hodder Advanced Databases
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A Level Computing for AQA Teachers Resource CD-ROM
2005 Bob Reeves, Dave Fogg/Hodder Murray
Home work
Research and write a at least 1 paragraph on the following
subjects:
Data warehousing
Data mining
using examples from supermarkets and insurance companies.
Prepare for end of sub-unit test