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Stuff iThink that will be on the Quiz/Midterm. Fourth Normal Form. by Andrew Yee. Why 4NF?. Performance We want database to take up less space. Why 4NF?. To eliminate the possibility for data anomalies. Why 4NF?. To reduce the need for: inserts, updates deletes.

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## Stuff iThink that will be on the Quiz/Midterm.

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**Stuff iThinkthat will be on the Quiz/Midterm.**Fourth Normal Form by Andrew Yee**Why 4NF?**• Performance • We want database to take up less space.**Why 4NF?**• To eliminate the possibility for data anomalies.**Why 4NF?**• To reduce the need for: • inserts, • updates • deletes**Non-Trival Multi-Valued Dependencies (MVD)**• Assume X is the Superkey • “Superkey” still depends on FD’s only. • Y is not a subset of X • X and Y are not, together, all the attributes.**Recap of 4NF Definition**• No non-trivial multi-valued dependencies - Except on superset of a candidate key - Multi-valued dependencies are functional dependencies. • Table must be in BCNF**Recap of 4NF Definition**• No Duplications • Table must be in BCNF**Example to get 4NF**• Assume the table is in BCNF • The town, Region, and District together form the candidate key. From Reference #1**Example to get 4NF**• Decomposed into two tables From Reference #1**Example 2: Is it in 3NF or BCNF?**Pizza Delivery From Reference #2**Example 2: Is it in 3NF or BCNF?**• The table has no non-key attributes because its only key is {Restaurant, Pizza Variety, Delivery Area}. • The answer is BCNF. From Reference #2**Example 2: Is it in 4NF?**Pizza Delivery From Reference #2**Example 2: Is it in 4NF?**• If we assume, that pizza varieties offered by a restaurant are not affected by delivery area, then it does not meet 4NF. • The answer is NO. From Reference #2**Why not in 4NF?**Two Non-trivial: MVD Pizza Delivery From Reference #2**Why not in 4NF?**• The table has two non-trivial multivalued dependencies on the {Restaurant} attribute (which is not a superkey). • The dependencies are: • {Restaurant} →→ {Pizza Variety} • {Restaurant} →→ {Delivery Area} From Reference #2**How to “fix it” (4NF)?**Two Non-trivial: MVD Pizza Delivery From Reference #2**How to “fix it” (4NF)?**Varieties By Restaurant Delivery Areas By Restaurant No Duplications among rows! Yay! From Reference #2**Practice Example**• Is this 3NF or BCNF? • Is this 4NF? • If not, what are the decomposed tables in 4NF? Pizza Delivery**Practice Example (Answer)**• BCNF 2. NO 3. See Below Delivery Areas By Restaurant Varieties By Restaurant No Duplications among rows! Yay!**References**• Website: Database Design: 4th and 5th Normal Forms « Tod means Fox URL:http://blog.todmeansfox.com/2007/12/04/database-design-4th-and-5th-normal-forms/ 2. Website: Fourth normal form - Wikipedia, the free encyclopedia URL: http://en.wikipedia.org/wiki/Fourth_normal_form

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