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92 lines
2.8 KiB
Markdown
92 lines
2.8 KiB
Markdown
---
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title: "dependencies-among-attributes"
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aliases: dependencies among attributes
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tags:
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- info201
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---
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# Functional Depenencies (FDs)
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If for any given value of attribute A there is _exactly one_ associated value of attribute B, then A _functionally determines_ B (loosely)
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This is the theoretical basis for [normalisation](notes/normalisation.md), and uniqueness property of PK (A is unique with respect to B)
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- one to one
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- Written as: A --> B
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- Equivalently, "B is functionally dependent on A"
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- Within a single relation only
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- every attribute functionally dependent of primary key (PK)
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## Example 1
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- consdier a specific student ID e.g., 123346
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- this student ID is alwasys associated witha single studnet name (e.g., jane smith)
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- even it the students name changes, that student ID will still be asociated with the name of only that on student
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- *The value of student id determines the value of student name*
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## Other examples
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- student ID --> student name (but not vice versa)
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- car registration --> car owner (but not vice versa)
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- rego --> VIN
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- VIN --> rego
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- student ID --> name, semester address, mobile number
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- car rego --> owener name
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- IRD number + year --> tax payable
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- product ID + order no --> quantity ordered
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## Anti examples
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- student ID + name --> birth date (overkill, partial dependency)
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- home address --> student name
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- name --> birth date
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e.g.,
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## Using Functional dependencies
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To determine them:
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- need detailed knowledge of thebusiness rules
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- examine existing data sets
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- not always practical when these are large or unknown
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Can be represented using funcitonal dependency diagrams (FDDs)
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Bottom up approach
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- ERD is "top-down"
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- FD best used as a design validation tool
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## Types of functional dependencies
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### Dependencies on more that one attribute
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non primary attributes that are dependent on two or more attributes
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always arise with composite PKs
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e.g.,
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### Partial Dependency
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Subset of left hand side determines right hand side
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"extra attributes"
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e.g.,
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### Transitive dependency
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e.g.,
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- part num determines supplier number
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- supplier number determines supplier name
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- part number determines supplier name
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BUT 3 is already implied by 1 & 2 --> redundant supplier names
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### Multivalued dependencies (MVDs)
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if for any given value of attribute A there is a _set_ of associated values of attribute S, the a _Multidetermines_ S (loosely)
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- one to many
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- written: A ↠ S
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- equivalently, "S is multiply dependent on A"
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- Generalistion of FDs: all FDs are MVDs, but not vice versa
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- A is still unique with respect to S
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