Relational Databases9IST Information Software Technology

T3L7 - Introducing relationships in DBMS

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About this show

• Learn about

• Using multiple tables in a DBMS

• Saving time and being flexible

• Advantages of Relational Databases over Flat File Databases

• Describing a Relational Database

Relationships

• a way in which people and things are connected to each other

• husband and wife• father and son• brother and sister• transmission and engine

Relational DBMS

• When a Database contains facts that are organised into more than one table

• If the facts have some ‘relationship’ that can be used to produce information as a result of BOTH tables

• The resulting information is generated because of a relationship that the designer has made.

2 Table Example

• Table 1 : The student table - contains facts about a student (name, address, year etc)

• Table 2 : The class table - contains facts about classes in school (year, subject)

• If we want to generate a report about which students are in which class we could take advantage of a RELATIONAL DATABASE

What if we use 1 table?

• If we use 1 table, we have to manually enter the names of the classes the students are for EACH student.

• Requires more storage, takes longer and causes REDUNDANCY

• REDUNDANCY - saying things over and over when you could say them once.

Relational tablesStudent tbl Class tbl Year tbl

Name (text)AddressYear

artd&tcomputingmathsenglishgeographyhisepdhpe

789101112Using 3 tables that

are related - we can ‘pull’ information together that we need to form the class information

Why do this?

• Because we have 1 simple table to manage the facts (data) about the possible years

• 1 simple table about the subjects.

• We can add, change or delete subjects from this table really easily - and use these facts in other places too

• We can keep a simple record of a student and use these facts in other places too

Why do this?

• Having data broken up into smaller, simpler tables makes it easier to manage and more flexible.

• we dont need to repeat ourselves over and over and so use less space and less time

• If a table becomes corrupt - it is easier to RESTORE (fix)

Creating a class table

• What fields to we need to create a class table?

• name/year/subject

• so we can generate this table by ‘looking up’ facts from the first 3 we made.

• this is called a LOOKUP

• Look up the stuff you want and put it in

Finding who’s who?

• This is done in s DBMS by asking questions

• Asking questions is called QUERYING

• If theres something wrong with a price at the shops, people often ‘query’ the bill with the cahier?

• Does the store have what I want - you ask a member of staff - this too is a QUERY.

Querying• The question we would need to ask the

DBMS for a class list is simple

• What STUDENTS are Maths

• SELECT names WHERE class = Maths

• what information would you get if you asked these ...

• SELECT names WHERE class <> Maths

• SELECT names WHERE class = Maths and year = 9

Summary

• Relational Databases are

• Good ways to break down ‘facts’ that can be re-used to remove REDUNDANCY

• Effective ways to asks questions - QUERY the data to produce new information

• Lower storage needs and increase speed

• Guard against terminal ‘data loss’

Activity

• Using a video camera in groups of 4

• Think about a ‘role play’ in which your group can describe the idea of a RELATIONAL DATABASE - perhaps each person plays the role of a table and someone else pretends to be the database manager.

• Role play how a query works.

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Watch This!

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