35 lines
990 B
Markdown
35 lines
990 B
Markdown
# lec22
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## Functional Dependancy
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If we have an attribute `a` that could produce `b,c,d` reliably everytime then we would only need to keep track of `a` instead of keeping track of all the repeats because the dependants depend on `a`.
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Example:
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```
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building -> roomNumber
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```
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This one makes sense since we would likely be able to find duplicate room numbers in different buildings.
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Most places have a name for each building however.
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## BCNF
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> Boyce Codd Normal Form
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If we have a key which sources our redundancy we're ok because we can easily find all redundancies.
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If we're not looking at a key then we could be in the middle of a dependancy nest.
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Say we have a schema like `advisees(inst_id, student_id, student_name)`.
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Student name depends on `student_id` so there's no reason to have it in the schema.
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A schema follows BCNF if there's a non-trivial functional dependancy
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* x -> y for r
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* then x is a superkey for r
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## Example
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```
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instructor_offices(inst_id, name, office)
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```
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