Transcript Introduction to databases

Databases
What is a database?
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A database is a collection of data
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Maybe computerised
Will be organised so as to make the retrieval and
maintenance of data as easy as possible
A computerised database:
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Offers the great advantage of speed
More sophisticated databases will use database
management software
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MS Access is an RDBMS – Relational Database Management
System
Flat files
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A flat file is a database held in a single file
The data stored in one file cannot easily be linked to
data stored in another file.
These so-called ‘flat files’ are useful for:
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a list of contact names and addresses
a list of student grades
details of a video collection.
A spreadsheet can be used as a flat file database,
with a column representing a field and a row
representing a record.
Flat file problems
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Redundant data – repetition of the same data
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Errors arising from typing the same data many
times in different records
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makes the file larger than necessary
wastes space on disk
makes it slow to retrieve information
Typographical errors occur
Loss of data consistency
Difficulties in updating and modifying data
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All occurrences would have to be located and changed
Solving the flat file problems
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Study this flat file data for a moment
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Identify some of the problems that have occurred
Consider how multiple tables would improve the
database for this orders flat file
Multiple tables
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Each entity should have its own table
Each record must have a unique ID.
This is the principle behind relational databases.
The following tables would be created from the previous flat file
Multiple tables
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This would then mean that the orders table would look like:
 In reality, the supplier data in the Products table would be
regarded as a separate entity and have its own table
 Why is price retained as a field in the orders table?
Database system .v. flat file
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data independence
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quality of management information
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information more useful as it is based on a single, comprehensive set of data
control over redundancy
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structure of the database does not affect the programs which access the
database
data is not stored several times in separate files
consistency of data
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because data is only stored once, no possibility of holding different data on
different files (e.g. different addresses for the same employee)
Database system .v. flat file (cont.)
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more information available to users
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greater security of data
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because the data is held in a single, company-wide database, all
users have access to it
users can only view or update the parts they are authorised to
access or change
less time spent inputting data
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because it is held only once, there is no duplication of effort
inputting the same data into different files.