The big idea: Normalisation rearranges tables so every fact is stored in exactly one place.
A fact stored twice can end up with two different values. A fact stored once cannot.
The word that runs through all of it: A functional dependency means one column's value determines another's.
Know a StudentID and you know the student's name: Name depends on StudentID. Normal forms are rules about which dependencies are allowed where.
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One school table taken from unnormalised to 3NF, with the rule and the problem named at each step.
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1NF — atomicity
2NF — no partial-key dependencies
3NF — no transitive dependencies
A one-line test for each: 1NF — is any cell holding a list? 2NF — does anything depend on only part of a composite key? 3NF — does any non-key column depend on another non-key column?
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Update problems
- A teacher's room is written on every row they appear on
- Change it and you must find every copy
- Miss one and the database contradicts itself
Insert problems
- A new club with no members yet has nowhere to live
- You cannot record a club without inventing a fake student
- Missing data where the design forces it
Delete problems
- Remove the last student in a club
- The club's teacher and room disappear with them
- Data lost that nobody meant to delete
Why these three are the argument: "It saves space" is the weakest reason and rarely worth a mark.
The real case is that an unnormalised table makes it possible to be wrong — and a normalised one does not.
How this is tested — you must name the form, state its rule, and identify the offending dependency. It comes up two ways:
Paper 1 Section A
- Explain the difference between normal forms, 3-5 marks
- State which form a given table is in, and why
- Define a named dependency
Paper 1 Section B — case study
- Assess the case study's table design
- Explain the problems its current shape causes
The classic trap: Saying a table "is not in 3NF" without naming which dependency breaks it. The mark is for identifying the offending columns: Teacher determines Room, and neither is a key.
A table stores OrderID, ProductID, Quantity, ProductName and SupplierAddress, keyed on OrderID and ProductID. Explain which normal forms it breaks.
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