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Lesson Objectives Aims You should know about: 1.3.2:
(a) Relational database, flat file, primary key, foreign key, secondary key, entity relationship modelling, normalisation and indexing.
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Some key terms Indexing:
An index is a data structure used to shorten the length of time it takes to search a database. An index may point to other sub indexes Because these structures are smaller than the whole database it increases search speed
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Normalisation Normalisation is the process of converting a flat file database (with a single table) to a relational database (with many tables). There are various levels of normalisation that remove repetition to a greater or lesser extent.
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Normalisation Weird. Because logically, you’d never put data in to the first normal form… But they are clear, well defined steps. Read and make notes on the “Normalisation to 3NF” in the shared area
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Task Step 1: Turn the un-normalised data in to 1NF
PupilID PupilName DOB ExamID Subject Level Date RoomID RoomName P99010 Jane Grey CP101 EN004 AR075 Computing English Art AS GCSE UH UG Hall Gym P99205 Tom Jones MA110 PH190 Maths Physics 58 Science Lab P99311 Sam Hill FIRST STEP: To change into First Normal Form – the repeated groups of fields must go. To do this, data must be split in to separate tables
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1st Normal Form A table is in First Normal Form (1NF) if there are no repeating groups. In other words, each column must contain only a single value and each row must have an item in every column. This can usually be done by putting the data into two tables ... separating the repeated data into a separate group.
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1NF PupilID PupilName DOB ExamID P99010 Jane Grey 12.03.86 CP101 EN004
AR075 P99205 Tom Jones MA110 PH190 P99311 Sam Hill ExamID Subject Level Date RoomID RoomName CP101 Computing AS UH Hall AR075 Art UG Gym MA110 Maths PH190 Physics 58 Science Lab EN004 English GCSE
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To move to 2NF, any partial dependencies must be removed
This basically means each record should not have a composite primary key This removes: Many to many relationships Repeated Data
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In the first table, there is a composite key (PupilID and ExamID)
Many to many In the first table, there is a composite key (PupilID and ExamID) There is also a lot of data repetition (many students taking many exams) PupilID PupilName DOB ExamID P99010 Jane Grey CP101 EN004 AR075 P99205 Tom Jones MA110 PH190 P99311 Sam Hill
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Second Normal Form PupilID PupilName DOB P99010 Jane Grey 12.03.86
Tom Jones P99311 Sam Hill PupilID ExamID P99010 CP101 EN004 AR075 P99205 MA110 PH190 P99311 ExamID Subject Level Date RoomID RoomName CP101 Computing AS UH Hall AR075 Art UG Gym MA110 Maths PH190 Physics 58 Science Lab EN004 English GCSE
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3rd Normal Form removes something called “Transitive Dependency”
Third Normal Form 3rd Normal Form removes something called “Transitive Dependency” The advantage of removing transitive dependency is: Amount of data duplication is reduced. Data integrity achieved.
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What on earth is transitive dependency?
Basically it means all data in the table should be dependent solely on the primary key. Any other data should be in a new table
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In our 2NF table, ExamID is the PK
However, RoomName does NOT depend on ExamID, it is dependent on RoomID. Therefore this data should be in a new table ExamID Subject Level Date RoomID RoomName CP101 Computing AS UH Hall AR075 Art UG Gym MA110 Maths PH190 Physics 58 Science Lab EN004 English GCSE
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PUPILS (PupilID, PupilName, DOB)
Jane Grey P99205 Tom Jones P99311 Sam Hill PupilID ExamID P99010 CP101 EN004 AR075 P99205 MA110 PH190 P99311 ExamID Subject Level Date RoomID CP101 Computing AS UH AR075 Art UG MA110 Maths PH190 Physics 58 EN004 English GCSE RoomID RoomName UH Hall UG Gym 58 Science Lab PUPILS (PupilID, PupilName, DOB) EXAMS (ExamID, Subject, Level, Date, RoomID) PUPIL_SITS (PupilID, ExamID) ROOMS (RoomID, RoomName)
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Review/Success Criteria
You should know: How to normalise data The definitions of the three normal forms The purpose of normalisation
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