normalization forms - intro to db

8/6/2019 Normalization Forms - Intro to Db

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Well-Structured Relations A relation that contains minimal data redundancy

and allows users to insert, delete, and update rowswithout causing data inconsistencies

Goal is to avoid anomalies

Insertion Anomaly adding new rows forces user tocreate duplicate data

Deletion Anomaly deleting rows may cause a loss ofdata that would be needed for other future rows

Modification Anomaly changing data in a row forceschanges to other rows because of duplication

General rule of thumb: a table should not pertain to

more than one entity type


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Example Figure 5.2b

Question Is this a relation? Answer Yes: unique rows and no multivaluedattributes

Question Whats the primary key? Answer Composite: Emp_ID,Course_Title


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Anomalies in this Table

Insertion cant enter a new employee without

having the employee take a class

Deletion if we remove employee 140, we lose

information about the existence of a Tax Acc class

Modification giving a salary increase to

employee 100 forces us to update multiple records

Why do these anomalies exist?

Because weve combined two themes (entity types)

into one relation. This results in duplication, and an

unnecessary dependency between the entities


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Functional Dependencies and Keys

Functional Dependency: The value of oneattribute (the determinant) determines thevalue of another attribute

Candidate Key: A unique identifier. One of the candidate keys

will become the primary key

E.g. perhaps there is both credit card number andSS# in a tablein this case both are candidate keys

Each non-key field is functionally dependent onevery candidate key


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5.22 -Steps in

normalization


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First Normal Form

No multivalued attributes

Every attribute value is atomic

Fig. 5-2a is notin 1st

Normal Form(multivalued attributes) it is not arelation

Fig. 5-2b is in 1st

Normal form All relations are in 1st Normal Form


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Second Normal Form

1NFplus every non-key attribute is fullyfunctionally dependent on the ENTIREprimary key

Every non-key attribute must be defined by theentire key, not by only part of the key

No partial functional dependencies

Fig. 5-2b is NOT in 2nd

Normal Form (seefig 5-23b)


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Fig 5.23(b) Functional Dependencies

in EMPLOYEE2

EmpID CourseTitle DateCompletedSalaryDeptNameName

Dependency on entire primary key

Dependency on only partof the key

EmpID, CourseTitle DateCompletedEmpID Name, DeptName, Salary

Therefore, NOT in 2nd Normal Form!!


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Getting it into 2ndNormal Form

See p193 decomposed into two separaterelations

EmpID SalaryDeptNameName

CourseTitle DateCompletedEmpID

Both are fullfunctional

dependencies


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Third Normal Form 2NF PLUS no transitive dependencies

(one attribute functionally determines a

second, which functionally determines a

third)

Fig. 5-24, 5-25


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Figure 5-24(b) Relation with transitive dependency

CustID Name

CustID SalespersonCustID Region

All this is OK

(2nd NF)

BUTCustID Salesperson Region

Transitive dependency

(not 3rdNF)


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Figure 5.25 -- Removing a transitive dependency

(a) Decomposing the SALES relation


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Figure 5.25(b) Relations in 3NF

Now, there are no transitive dependencies

Both relations are in 3rd NF

CustID Name

CustID

Salesperson

Salesperson Region


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Other Normal Forms

(from Appendix B) Boyce-Codd NF

All determinants are candidate keysthere is no determinantthat is not a unique identifier

4thNF No multivalued dependencies

5thNF

No lossless joins

Domain-key NF The ultimate NFperfect elimination of all possible

anomalies

normalization forms - intro to db

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