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Flip to reveal answersWhat are the three database schema levels?
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Question
What are the three database schema levels?
Answer
Conceptual — the entities and relationships the organisation cares about. Logical — those turned into tables, columns, keys and data types. Physical — how the chosen system actually stores it, including files and indexes.
Question
What does a conceptual schema contain, and what does it leave out?
Answer
It contains the entities and the relationships between them, in language the organisation recognises. It deliberately omits keys, data types and any choice of database product.
Question
What belongs to the physical schema?
Answer
How the chosen system stores the data: files on disk, indexes, partitions and storage settings. This is where performance decisions live, and it must be redone if you change database product.
Question
What is data independence?
Answer
The schema levels are insulated from each other, so a change at one does not force a change at another. Adding an index at the physical level alters no query written against the logical schema.
Question
Why describe one database three times?
Answer
So the meaning can be agreed with non-technical people before technical choices are made, so storage can change without breaking programs, and so the same logical design can be built on different database systems.
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Topic 3.2 hub
Database design
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