Week 5 (Wednesday and Friday)

Week 5 (Wednesday and Friday) - Normalization Getting rid...

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8/3/2009 1 Normalization Getting rid of data anomalies Session 5 Checkpoint y We know that Data redundancy is bad Designing an effective database requires proper methodologies and tools 5-2 y You have a basic idea of the relational model y You are at least familiar with ER modelling So what’s next? y You have understood the business rules and captured them in the ER diagram In practice, this is an iterative process Refine with customers, programmers, etc 5-3 y Converting from ER diagram to tables involves a process called normalization Simply another database design tool
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8/3/2009 2 What is normalization? y A process to evaluate and correct table structures Goal is to eliminate data redundancies why? 5-4 y Process is mechanical Goes through a number of stages Each stage is a “normal form” Example y BNG manages building projects Charges clients by billing hours spent on each contract Hourly rate is dependent on the employee’s 5-5 classification y Purpose of BNG’s data To generate a report about each project Also auxiliary data reporting Normalization y First stage is called the first normal form (1NF) y Generally, (n+1)NF is better than nNF y For most situations, 3NF is the furthest 5-6 we go y Higher normal forms have less redundancies Not always desirable why?
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8/3/2009 3 Example (cont.) cont.) 5-7 Example (cont.) cont.) 5-8 What’s the problem? First normal form (1NF) y A relation (or table) must not contain repeating groups A group of multiple entries can exist for any single key attribute occurrence 5-9 Goal of 1NF is to get rid of repeating groups y Informally A “key” should not identify a group of records or rows A “key” should identify one record
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8/3/2009 4 A table not in 1NF 5-10 A table not in 1NF A table in 1NF y Achieve 1NF by: Removing null entries Identifing the attribute(s) that make up a primary key 5-12 All other attributes in the table should be dependent on the primary key
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8/3/2009 5
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