Reln Database

8/8/2019 Reln Database

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Chapter 3 The Relational Database

Model

l Tthe relational database model takes a logical view of datal The relational model's basic components are entities, attributes, and relationships among

entitiesl How entities and their attributes are organized into tablesl About relational database operators, the data dictionary, and the system catalogl How data redundancy is handled in the relational database modell Why indexing is important

A logical View of Data

l Relational model Enables us to view data logically rather than physically Reminds us of simpler file concept of data storage or spreadsheet concept of table

l Table Two-dimensional structure composed of rows and columns Has advantages of structural and data independence Resembles a file from conceptual point of view Easier to understand than its hierarchical and network database predecessors

Tables

l Table also called a relation because the relational model's creator, Codd, used this term.l A relation (and a relational DBMS's table) differs from a table in that there is no default order

of rows or columns Note SQL does use a column order but this is a limitation of SQL, not the model.

l Each row (tuple) represents a single entity in an entity setl Each column represents an attribute and each must have a distinct namel Each row/column intersection (cell) represents a single atomic valuel All values in a column must have same data formatl Each column has a specific range of values known as a domain l Each row must have some combination of attributes that uniquely identifies it (key)

Student Table Example

Page 1 of 18Chapter 3 Notes

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Keys

l A key consists of one or more attributes that determine other attributesl Primary key (PK) is an attribute (or a combination of attributes) that uniquely identifies any

given entity (row)l Key's role is based on determination

If you know the value of attribute A, you can look up (determine) the value of attribute B

Types of Keys

Composite keyKey consisting of multiple attributes

Key attribute




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Any attribute that is part of a key

SuperkeyAny key that uniquely identifies each entityThe entire row (all attributes) is a superkey

Candidate keyA superkey without redundancies

Null Values

l Can represent An unknown attribute value A known, but missing, attribute value A 'not applicable' condition

l Not permitted in Primary key attributesl Not the same as 0 or ""l Can create problems in logic and using formulas

Controlled Redundancy

l Makes the relational database workl Tables within the database share common attributes that enable us to link tables togetherl Multiple occurrences of values in a table are not redundant when they are required to make the

relationship workl Uncontrolled Redundancy is unnecessary duplication of datal In general any values duplicated between tables should be part of a Foreign Key -- Primary key

relation (see below)

Foreign Keys

Foreign key (FK)An attribute whose values match primary key values in the related table

Referential integrityFK contains a value that refers to an existing valid tuple (row) in another relation

Secondary keyKey used strictly for data retrieval purposes

Keys (Summary)

Key Type Definition

Superkey Any attribute or combination of attributes that uniquely identifies a row in the table

CandiateKey

aka Minimal Superkey. A superkey that does not contain a subset of attributes that isitself a superkey

Primary Key The candidate key selected to uniquely identify all rowsCannot contain null values

Foreign KeyAn attribute (or combination of attributes) in one table that must either match the




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In tegrity Rules

Entity Integrity

l All entities are uniquel Each entity has unique primary key

Referential Integrity

l Foreign key must match primary key of referenced table or be nulll Impossible to delete row whose primary key has matching foreign key in another table

Relational Algebra

Key operators:

l SELECTl PROJECTl JOIN

Other operators

l INTERSECTl UNIONl DIFFERENCEl PRODUCTl DIVIDE

SELECT

Subset of rows based on condition

primary key of another table or be null

SecondaryKey

An attribute or combination of attributes used to make data retrieval more efficient.




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PROJECT

Subset of columns




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PRODUCT

All possible pairings of two tables




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JOIN

l Combines information from two or more tablesl Real power behind the relational database, allowing the use of independent tables linked by

common attributesl Join is actually a combination of Product, Select, and Project




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Natural JOIN

Links tables by selecting rows with common values in common attribute(s) (typically equality onforeign keys).

Three stage process:

l Product creates one tablel Select yields appropriate rowsl Project yields single copy of each attribute to eliminate duplicate columns

Natural J oin: Step 1 P roduct




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Natural Join Step 2: Select

Natural Join Step 3: P roject




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Natural J oin: Outcome

l Final outcome yields table that Does not include unmatched pairs Provides only copies of matches

l If no match is made between the table rows,

the new table does not include the unmatched row

Other Joins

EquiJOIN

l Links tables based on equality condition that compares specified columns of tablesl Does not eliminate duplicate columnsl Join criteria must be explicitly defined

Theta JOIN

l EquiJOIN that compares specified columns of each table

Outer JOIN

l Matched pairs are retainedl Unmatched values in other tables left nulll Right and left

Intersect




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Difference

Yields all rows in one table not found in the other tablethat is, it subtracts one table from the other

Union




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Divide

Requires user of single-column table and two-column table (I have never seen a DIVIDE)

Data Dictionary and System Catalog

Data Dictionary

l Metadatal Detailed account of all tables in database

System Catalog

l System-created databasel

Stores database characteristics and contentsl The database stores its own descriptionl Tables can be queried just like any other tablesl Automatically produces database documentation

Relationships w ithin Relational Database

Classified by cardinalities of entities participating

l 1:M Relational modeling ideal

Should be the norm in any relational database designl M:N

Must be avoided because they lead to data redundancies Split into a pari of 1:M Relationships

l 1:1 Should be rare

The 1:1 Relationship




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l One entity can be related to only one other entity, and vice versal Often means that entity components were not defined properlyl Could indicate that two entities actually belong in the same tablel Sometimes 1:1 relationships are appropriate to help avoid nulls

E-R Diagram

l Graphical representation of modell Chenl Crows Feet

ERD Symbols

l Rectangles represent entitiesl Chen:

diamonds represent relations numbers or m and n are cardinalities

l

Crows Foot Lines represent relationships bar crossing line is "1" side 3 pronged crows foot is many side

Example 1:M Relationship

Implementation of 1:M Relationship




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M:N Relationships

M:N Relationship




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l Cannot be directly implemented in a Relational DBMS

Linking Table

l Implementation of a composite entityl Yields required M:N to 1:M conversionl Composite entity table must contain at least the primary keys of original tablesl Linking table contains multiple occurrences of the foreign key valuesl Additional attributes may be assigned as needed

Composite ERD




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Linking Tables: Implementation




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Data Redundancy Revisited

Foreign keys control and reduce redundancy.

It is not that there is no redundancy in a properly designed database: the redundancy there is is

controlled.

Indexes

Note: I do not consider indexes part of the relational model. They are an implementationartifact to improve performance

l Arrangement used to logically access rows in a tablel Index key




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Indexs reference point Points to data location identified by the key

l Unique index Index in which the index key can only have one pointer value (row) associated with it

l Each index is associated with only one table

Notes


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