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56 lines
1.2 KiB
Markdown
56 lines
1.2 KiB
Markdown
---
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title: "databases"
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aliases:
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tags:
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- cosc203
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---
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# Modelling
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Database: integrated collection of data
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- meaning
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- specific purpose
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[ER Diagram](notes/ER-Diagram)
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[Schemas](notes/Schemas)
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# Relational Model
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Two parts
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- Relation Schema
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- Domain
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Constraints
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- Domain: each value must an atomic value in the domain e.g., integer, string, date, etc
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- Key and NULL:
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- Superkey: uniquely ids a tuple e.g., set of all attrs
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- Key: minimal superkey
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- Candidate key: a possible key
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- Primary key: the chosen candidate key: cannot have NULL values
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- Ref Integrity: Foreign key
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- Functional Dependencies
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- fd: when one value determines another e.g., sid -> sname, {sid, cid} -> grade
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- armstrong axioms: 
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# Normal Forms
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Help to prevent anomalies (update, delete, insert) by reducing redundancy
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e.g.,
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```
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bad:
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Student_Course(sid, sname, address, cid, cname, grade)
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good:
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Student(sid,sname,address)
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Course(cid,cname)
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nrolled(sid,cid,grade)
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```
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done through process of decomposition: breaking down schema into set of schemas while preserving:
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- attributes
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- dependencies
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- lossless joins
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Various degrees of normalisatin defined by normal forms:
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[Normal Forms](notes/Normal-Forms)
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# SQL
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[SQL](notes/SQL) |