The big idea: A schema is a description of how the data is structured.
The same database is described three times, at three levels of detail — because three different audiences need three different amounts.
Think of a building: A sketch of the rooms, a technical drawing with dimensions, and the builder's specification of materials.
All three describe one building. None is wrong; each is aimed at someone different.
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1
Conceptual schema — what the data is
2
Logical schema — how it is organised
3
Physical schema — how it is stored
What changes between them: Going down the levels, each step adds detail and ties you to a decision.
Conceptual survives any technology. Logical survives any particular product. Physical is specific to what you actually installed.
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What the separation buys
- You can agree the meaning with non-technical people before any technical choices
- Changing how data is stored does not change how programs query it
- The same logical design can be built on different database systems
- Performance can be tuned at the physical level without redesigning anything above
What happens without it
- Technical detail is decided before anyone agrees what the data means
- A storage change breaks every program that reads the database
- The design is welded to one product from the start
- Tuning performance means redesigning the tables
The phrase for this: Data independence — the levels are insulated from each other, so a change at one does not force a change at another.
That is the whole reason three schemas exist rather than one.
How this is tested — you must say what each level contains and what it deliberately leaves out. It comes up two ways:
Paper 1 Section A
- Describe the three schemas, 3-4 marks
- Distinguish two of them
- Say which level a stated decision belongs to
Paper 1 Section B — case study
- Identify the levels in the case study's design
- Say which level a proposed change affects
The classic trap: Describing the levels as "more and more detailed" without saying what detail. The marks come from naming the contents: entities and relationships, then tables and keys, then files and indexes.
IB-style questionDescribe[4 marks]
Describe the three levels of database schema, using a school's club membership system as your example.
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