Normalization.pptx Functional dependence

Functional Dependencies and
Normalization for Relational
Databases
Dr. Ali Obaidi
CS-450 Fall 2002

Informal Design Guidelines for
Relational Databases
 Relational database design: The grouping of
attributes to form "good" relation schemas
 Two levels of relation schemas:
 The logical "user view" level
 The storage "base relation" level
 Design is concerned mainly with base relations
 Criteria for "good" base relations:
 Discuss informal guidelines for good relational design
 Discuss formal concepts of functional dependencies and
normal forms 1NF 2NF 3NF BCNF

Semantics of the Relation
Attributes
 Each tuple in a relation should represent one entity
or relationship instance
 Only foreign keys should be used to refer to other
entities
 Entity and relationship attributes should be kept apart as
much as possible
 Design a schema that can be explained easily relation by
relation. The semantics of attributes should be easy to
interpret.

Redundant Information in
Tuples and Update Anomalies
 Mixing attributes of multiple entities may
cause problems
 Information is stored redundantly wasting
storage
 Problems with update anomalies:
 Insertion anomalies
 Deletion anomalies
 Modification anomalies

EXAMPLE OF AN UPDATE
ANOMALY
Consider the relation:
EMP_PROJ ( Emp#, Proj#, Ename, Pname, No_hours)
 Update Anomaly
 Changing the name of project number P1 from “Billing” to
“Customer-Accounting” may cause this update to be made for all
100 employees working on project P1
 Insert Anomaly
 Cannot insert a project unless an employee is assigned to .
 Inversely- Cannot insert an employee unless he/she is assigned to
a project.

EXAMPLE OF AN UPDATE
ANOMALY (2)
 Delete Anomaly
 When a project is deleted, it will result in deleting all the
employees who work on that project. Alternately, if an employee
is the sole employee on a project, deleting that employee would
result in deleting the corresponding project.
 Design a schema that does not suffer from the
insertion, deletion and update anomalies. If there
are any present, then note them so that applications
can be made to take them into account

Null Values in Tuples
 Relations should be designed such that their tuples
will have as few NULL values as possible
 Attributes that are NULL frequently could be placed in
separate relations (with the primary key)
 Reasons for nulls:
 a. attribute not applicable or invalid
 b. attribute value unkown (may exist)
 c. value known to exist, but unavailable

Spurious Tuples
 Bad designs for a relational database may result in
erroneous results for certain JOIN operations
 The "lossless join" property is used to guarantee
meaningful results for join operations
 The relations should be designed to satisfy the
lossless join condition. No spurious tuples should
be generated by doing a natural-join of any
relations

Functional Dependencies
 Functional dependencies (FDs) are used to
specify formal measures of the "goodness"
of relational designs
 FDs and keys are used to define normal
forms for relations
 FDs are constraints that are derived from
the meaning and interrelationships of the
data attributes

Functional Dependencies (2)
 A set of attributes X functionally determines a set of
attributes Y if the value of X determines a unique value for
Y
 X Y holds if whenever two tuples have the same value for
X, they must have the same value for Y
If t1[X]=t2[X], then t1[Y]=t2[Y] in any relation instance r(R)
 X  Y in R specifies a constraint on all relation instances
r(R)
 FDs are derived from the real-world constraints on the
attributes

Examples of FD constraints
 Social Security Number determines employee name
SSN  ENAME
 Project Number determines project name and
location
PNUMBER  {PNAME, PLOCATION}
 Employee SSN and project number determines the
hours per week that the employee works on the
project
{SSN, PNUMBER}  HOURS

Functional Dependencies (3)
 An FD is a property of the attributes in the
schema R
 The constraint must hold on every relation
instance r(R)
 If K is a key of R, then K functionally
determines all attributes in R (since we never
have two distinct tuples with t1[K]=t2[K])

Inference Rules for FDs
 Given a set of FDs F, we can infer additional FDs
that hold whenever the FDs in F hold
 Armstrong's inference rules
A1. (Reflexive) If Y subset-of X, then X  Y
A2. (Augmentation) If X  Y, then XZ  YZ
(Notation: XZ stands for X U Z)
A3. (Transitive) If X  Y and Y  Z, then X  Z
 A1, A2, A3 form a sound and complete set of
inference rules

Additional Useful Inference
Rules
 Decomposition
 If X  YZ, then X  Y and X  Z
 Union
 If X  Y and X  Z, then X  YZ
 Psuedotransitivity
 If X  Y and WY  Z, then WX  Z
 Closure of a set F of FDs is the set F+ of all FDs
that can be inferred from F

Introduction to
Normalization
 Normalization: Process of decomposing
unsatisfactory "bad" relations by breaking up their
attributes into smaller relations
 Normal form: Condition using keys and FDs of a
relation to certify whether a relation schema is in a
particular normal form
 2NF, 3NF, BCNF based on keys and FDs of a relation
schema
 4NF based on keys, multi-valued dependencies

First Normal Form
 Disallows composite attributes, multivalued
attributes, and nested relations; attributes
whose values for an individual tuple are
non-atomic
 Considered to be part of the definition of
relation

Second Normal Form
 Uses the concepts of FDs, primary key
 Definitions:
 Prime attribute - attribute that is member of the
primary key K
 Full functional dependency - a FD Y  Z
where removal of any attribute from Y means the
FD does not hold any more

Examples
Second Normal Form
 {SSN, PNUMBER}  HOURS is a full FD since neither
SSN  HOURS nor PNUMBER  HOURS hold
 {SSN, PNUMBER}  ENAME is not a full FD (it is
called a partial dependency ) since SSN  ENAME also
holds
 A relation schema R is in second normal form (2NF) if
every non-prime attribute A in R is fully functionally
dependent on the primary key
 R can be decomposed into 2NF relations via the process
of 2NF normalization

Third Normal Form
 Definition
 Transitive functional dependency – if there a set of
atribute Z that are neither a primary or candidate key and
both X  Z and Y  Z holds.
 Examples:
 SSN  DMGRSSN is a transitive FD since
SSN  DNUMBER and DNUMBER  DMGRSSN hold
 SSN  ENAME is non-transitive since there is no set
of
attributes X where SSN  X and X  ENAME

3rd
Normal Form
A relation schema R is in third normal form
(3NF) if it is in 2NF and no non-prime
attribute A in R is transitively dependent on
the primary key

BCNF (Boyce-Codd Normal
Form)
 A relation schema R is in Boyce-Codd Normal
Form (BCNF) if whenever an FD X  A holds in
R, then X is a superkey of R
 Each normal form is strictly stronger than the previous
one:
 Every 2NF relation is in 1NF
 Every 3NF relation is in 2NF
 Every BCNF relation is in 3NF
 There exist relations that are in 3NF but not in BCNF
 The goal is to have each relation in BCNF (or 3NF)

BCNF
 {Student,course}  Instructor
 Instructor  Course
 Decomposing into 2 schemas
 {Student,Instructor} {Student,Course}
 {Course,Instructor} {Student,Course}
 {Course,Instructor} {Instructor,Student}

Example
 Given the relation
Book(Book_title, Authorname, Book_type,
Listprice, Author_affil, Publisher)
The FDs are
Book_title  Publisher, Book_type
Book_type  Listprice
Authorname Author_affil

Example
 What normal form the relation in?

Normalization.pptx Functional dependence

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