Normalization of Database Tables

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Topics

• Role of normalization in the database design process

• Normal forms 1NF, 2NF, 3NF, BCNF, 4NF• Normalization & ER modeling used together for

good database design• Denormalization

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Normalization

• Process for evaluating/correcting table structures to minimize data redundancies– Reduces data anomalies

• Series of stages called normal forms – 1NF, 2NF, 3NF, BCNF, 4NF

• For many purposes, 3NF is good enough• Sometimes denormalization (to a lower normal

form) is advised

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Example

• A company manages building projects– Charges clients by hours spent on each contract

– Hourly billing rate depends on employee’s position

– Report: Table 5.1

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Report converted to table

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Goals of Normalization Process

• Each table represents a single subject• No data item will be unnecessarily stored in

more than one table• All attributes in a table are dependent only on

the primary key (or a candidate key)• Each table void of insertion, update, deletion

anomalies– All a product of data redundancy

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Conversion to First Normal Form

• Eliminate any repeating groups– They represent multi-valued attributes – bad!!!

– Eliminate nulls: each repeating group attribute contains an appropriate data value

• Identify the primary key (PK) – Must uniquely identify all attribute values

• Identify all dependencies – Dependencies depicted with a diagram

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Dependency Diagram

9What is the PK?

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Conversion to 1NF

• First normal form describes tabular format in which:– All key attributes are defined– There are no repeating groups in the table– All attributes are dependent on primary key

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Conversion to 2NF

• Eliminate partial dependencies– Only relevant if PK is composite

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Conversion to 3NF

• Eliminate transitive dependencies

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Improving the Design

• Issues to address beyond normalization – Evaluate PK assignments

– Evaluate naming conventions

– Refine attribute atomicity

– Identify new attributes

– Identify new relationships

– Refine primary keys for granularity

– Maintain historical accuracy

– Evaluate derived attributes

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Higher-Level Normal Forms

• Tables in 3NF perform suitably in business transactional databases

• Higher order normal forms useful on occasion• Two special cases of 3NF:

– Boyce-Codd normal form (BCNF)

– Fourth normal form (4NF)

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Boyce-Codd Normal Form (BCNF)

• Every determinant in table is a candidate key– Has same characteristics as PK, but was not

chosen to be primary key

• When table contains only one candidate key, 3NF and BCNF are equivalent

• Table can be in 3NF and fail to meet BCNF– Happens when a non-key attribute is the

determinant of a key attribute

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In 3NF but not BCNF

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In 3NF but not BCNF

PK is STU_ID, STAFF_ID

CLASS_CODE STAFF_ID

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Fourth Normal Form (4NF)

• Table is in 4NF when – It is in 3NF

– No multiple sets of multi-valued dependencies

• Not an issue if tables conform to these rules:– All attributes are dependent on PK, and are

independent of each other

– No row contains two or more multi-valued facts about an entity

• Example – see Fig. 6.10 & 6.11, pp. 196-197

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Normalization and Database Design

• Normalization is part of the design process• Proposed entities must meet required normal

form before table structures are created• Many real-world databases have been

improperly designed or burdened with anomalies

• Normalization is necessary if you are asked to redesign and modify existing databases

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Denormalization

• Creation of normalized relations is important database design goal

• Processing requirements should also be a goal– With normalization, the number of tables

expands

• Joining the larger number of tables reduces system speed

• Conflicts often resolved through compromises that may include denormalization`