Normalization for better DB deign

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Normalization for better DB deign

• MI375
• Sean Eom

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Normalization

• Normalization is a process for creating several
  tables from an un-normalized relation (Table).
  The purpose of normalization is to reduce data
  redundancies and eliminate the data anomalies.
• The techniques of normalization now provide a
  foundation for logical data base design.

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Data anomalies (An undesirable consequence of a
data modification) occurs with poorly designed
Tables.

• Insertion anomaly facts (data) about two or more
  different entities must be added to a single row
  of a relation.
• Deletion anomaly facts (data) about two or more
  different entities are lost when a single row of
  a relation is deleted.
• Update anomaly

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Insertion anomaly The table below contains info
about customers and their agents. To add new
customers to this table, we must enter the
corresponding agent data. By doing so, the
potential for creating data inconsistency is
great!
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Modification anomaly Using The same table below,
if agent Leah Hahn married to John Doe
yesterday, we need to change her name to Leah
Doe. In this case, only three changes must be
made. In a large table, such change might occur
in thousands of records. By doing so, the
potential for creating data inconsistency is
great!
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Deletion anomaly Using The same table below, if
agent Leah Hahn decides to quit the job because
John is a millionaire, we must delete all
fields in 3 records. In this case, only three
changes must be made. In a large table, such
change might occur in thousands of records. By
doing so, the potential for creating data
inconsistency is great!
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Normalization You have seen various types of
data anomalies with poorly designed tables, lets
design a better table using normalization. .

• Normalization is done through the analysis of
  functional dependencies between attributes (or
  data items or fields).
• The attribute B is functionally dependent on the
  attribute A if each value in column A determines
  one and only one value in column B.

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Steps in Normalization

• User views are identified.
• Each user view is converted to the form of an
  unnormalized relation. Any repeating groups are
  then removed from the unnormalized relation the
  result is a relation in first normal form (1NF).
• Any partial dependencies are removed from these
  relations the result is a set of relations in
  second normal form (2NF).
• Any transitive dependencies are removed, creating
  a set of relations in third normal form (3NF).

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Example 1

• Student_Activity REPORT

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User View

• The user view the data as if they were laid out
  in tabular form, which is an unnormalized
  relation (a relation that contains one or more
  repeating groups).
• Student_Activity REPORT is an example of
  user-view (next slide).

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User-view of student activity SID is a unique
student ID, Club is the name of a club, Cost is
the cost of joining a club, and AmtPaid is the
amount the student has paid to that club.
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An un-normalized relation user view SID is a
unique student ID, Club is the name of a club,
Cost is the cost of joining a club, and AmtPaid
is the amount the student has paid to that club.

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Remove repeating groups
Remove partial dependencies
Remove transitive dependencies
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Unnormalized Relations

• Student Activity REPORT is an example of an
  unnormalized relation.
• An unnormalized relation is a relation that
  contains one or more repeating groups.
• Repeating groups a group of several data
  entries associated with one attribute.
• Derives its name from the fact that a group of
  multiple (related) entries can exist for any
  single key attribute occurrence

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Conversion to First Normal Form

• A relational table must not contain repeating
  groups. The user-view of student activity report
  contains a repeating group, the 3rd row from the
  bottom.
• Repeating groups can be eliminated by adding the
  appropriate entry in at least the primary key
  column(s).

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Normalized Relations (1NF)
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• 1NF Definition
• The term first normal form (1NF) describes the
  tabular format in which
• There are no repeating groups in the table.
• (All attributes are dependent on the primary key)
• (All the key attributes are defined.)
• All relational tables satisfy the 1NF
  requirements.

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From 1NF to 2NF (As we remove partial dependency
from 1NF table, the table becomes 2NF).

• To do so, we need to draw a functional dependency
  diagram.
• A ----? B (This notation is interpreted as
• The attribute B is functionally dependent on the
  attribute A if each value in column A determines
  one and only one value in column B.
• The attribute A determines only one value in
  Column B.

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Functional Dependency analysis
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Functional Dependency Diagram
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Normalized Tables (2NF and 3NF)Each table is 3NF
table, which means it has no partial dependency
and transitive dependency.
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Example 2

• Example company that manages building projects
• Charges its clients by billing hours spent on
  each contract
• Hourly billing rate is dependent on employees
  position
• Periodically, a report is generated that contains
  information displayed in Table 5.1

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A Sample Report Layout
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A Table in the Report Format
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The Need for Normalization (continued)

• Structure of data set in Figure 5.1 does not
  handle data very well
• The table structure appears to work report is
  generated with ease
• Unfortunately, the report may yield different
  results, depending on what data anomaly has
  occurred

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Conversion to First Normal Form

• Repeating group
• Derives its name from the fact that a group of
  multiple (related) entries can exist for any
  single key attribute occurrence
• Relational table must not contain repeating
  groups
• Normalizing the table structure will reduce these
  data redundancies
• Normalization is three-step procedure

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Step 1 Eliminate the Repeating Groups

• Present data in a tabular format, where each cell
  has a single value and there are no repeating
  groups
• Eliminate repeating groups by eliminating nulls,
  making sure that each repeating group attribute
  contains an appropriate data value

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Data Organization First Normal Form
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Step 2 Identify the Primary Key

• Primary key must uniquely identify attribute
  value
• New key must be composed

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Step 3 Identify all Dependencies

• Dependencies can be depicted with the help of a
  diagram
• Dependency diagram
• Depicts all dependencies found within a given
  table structure
• Helpful in getting birds-eye view of all
  relationships among a tables attributes
• Use makes it much less likely that an important
  dependency will be overlooked

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A Dependency Diagram First Normal Form (1NF)
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First Normal Form

• Tabular format in which
• All key attributes are defined
• There are no repeating groups in the table
• All attributes are dependent on primary key
• All relational tables satisfy 1NF requirements
• Some tables contain partial dependencies
• Dependencies based on only part of the primary
  key
• Sometimes used for performance reasons, but
  should be used with caution
• Still subject to data redundancies

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Conversion to Second Normal Form

• Relational database design can be improved by
  converting the database into second normal form
  (2NF)
• Two steps

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Step 1 Identify All Key Components

• Write each key component on separate line, and
  then write the original (composite) key on the
  last line
• Each component will become the key in a new table

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Step 2 Identify the Dependent Attributes

• Determine which attributes are dependent on which
  other attributes
• At this point, most anomalies have been
  eliminated

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Second Normal Form (2NF) Conversion Results
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Second Normal Form

• Table is in second normal form (2NF) if
• It is in 1NF and
• It includes no partial dependencies
• No attribute is dependent on only a portion of
  the primary key

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Conversion to Third Normal Form

• Data anomalies created are easily eliminated by
  completing three steps

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Step 1 Identify Each New Determinant

• For every transitive dependency, write its
  determinant as a PK (Primary Key) for a new table
• Determinant
• Any attribute whose value determines other values
  within a row

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Step 2 Identify the Dependent Attributes

• Identify the attributes dependent on each
  determinant identified in Step 1 and identify the
  dependency
• Name the table to reflect its contents and
  function

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Step 3 Remove the Dependent Attributes from
Transitive Dependencies

• Eliminate all dependent attributes in transitive
  relationship(s) from each table that has such a
  transitive relationship
• Draw a new dependency diagram to show all tables
  defined in Steps 13
• Check new tables and modified tables from Step 3
  to make sure that each has a determinant and does
  not contain inappropriate dependencies

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Third Normal Form (3NF) Conversion Results
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Third Normal Form

• A table is in third normal form (3NF) if
• It is in 2NF and
• It contains no transitive dependencies

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A transitive dependency occurs when one nonkey
attribute (such as Charge_Hour in Fig. 5.4) is
dependent on one or more nonkey attributes (such
as Job_Class).
PRIMARY KEY
A
B

• In this case, there is a transitive dependency
  between the primary key and attributes A and B.
  Transitive dependencies result in insertion,
  deletion, and update anomalies, similar to those
  for partial dependencies.

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Improving the Design

• Table structures are cleaned up to eliminate the
  troublesome initial partial and transitive
  dependencies
• Normalization cannot, by itself, be relied on to
  make good designs
• It is valuable because its use helps eliminate
  data redundancies

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The Completed Database
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The Completed Database (continued)
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Summary

• Normalization is a table design technique aimed
  at minimizing data redundancies
• First three normal forms (1NF, 2NF, and 3NF) are
  most commonly encountered
• 2NF is better than 1NF 3NF is better than 2NF
• For most business database design purposes, 3NF
  is highest we need to go in the normalization
  process