Ch.8b Supplementary Note-relational Database

8/9/2019 Ch.8b Supplementary Note-relational Database

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Supplement for non-ISM students

By Tony WONG

A Logical View of DataA Logical View of Data

Relational database models structural and

data independence enables us to view data

logically rather than physically.

The logical view allows a simpler file

.

The use of lo icall inde endent tables is

easier to understand.

og ca s mp c ty y e s s mp er an more

effective database design methodologies.


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A Logical View of Data

Entities and Attributes

An entity is a person, place, event, or thing for which we intend

to collect data.n vers y -- u en s, acu y em ers, ourses

Airlines -- Pilots, Aircraft, Routes, Suppliers

Each entity has certain characteristics known as attributes.

Student -- Student Number, Name, GPA, Date of Enrollment,

, , ,

Aircraft -- Aircraft Number, Date of Last Maintenance, Total

Hours Flown, Hours Flown since Last Maintenance

A Logical View of Data

Entities and Attributes

A grouping of related entities becomes an entity set.

The STUDENT entity set contains all studententities.

The FACULTY entity set contains all faculty

entities.

The AIRCRAFT entity set contains all aircraft

entities.


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A Logical View of DataA Logical View of Data

Tables and Their Characteristics

A table contains a group of related entities -- i.e. an.

The terms entity set and table are often used

n erc angea y.

A table is also called a relation.

Summary of the Characteristics of a Relational Table


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A Listing of the STUDENT Table Attribute Values

KeysKeys

Primar Ke contains attribute s that can

uniquely identify one row in a table.

The primary key of one table appears again as the

.

Controlled redundancy (shared common

attributes) makes the relational database work.


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Figure 2.2 An Example of a Simple Relational Database

The Relational Schema for the CH2_SALE_CO Database


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Relational Database Keys

Relational Database OperatorsRelational Database Operators

Relational al ebra defines the theoretical

way of manipulating table contents using the

ei ht relational functions: SELECT,

PROJECT, JOIN, INTERSECT, UNION,DIFFERENCE, and PRODUCT.


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UNION combines all rows from two tables. The

two tables must be union compatible.


INTERSECT produces a listing that contains onlythe rows that appear in both tables. The two tablesmust be union compatible.


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DIFFERENCE yields all rows in one table that are not

found in the other table; i.e., it subtracts one table from the

other. The tables must be union compatible.


PRODUCT roduces a list of all ossible airs of

rows from two tables.


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SELECT yields values for all attributes found in a table. Itields a horizontal subset of a table.


PROJECT produces a list of all values for selectedattributes. It ields a vertical subset of a table.


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JOIN allows us to combine information from two

or more tables. JOIN is the real power behind the

relational database allowin the use of

independent tables linked by common attributes.

Relational Database Operators

Natural JOIN links tables by selecting only the rowswith common values in their common attribute(s). Itis the result of a three-stage process:

A PRODUCT of the tables is created. (Figure2.12)

A SELECT is performed on the output of the firststep to yield only the rows for which the common

. .

A PROJECT is performed to yield a single copy,

duplicate column. (Figure 2.14)


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Natural Join, Step 1: PRODUCT

Figure 2.13 Natural Join, Step 2: SELECT

Figure 2.14 Natural Join, Step 3: PROJECT

Ch.8b Supplementary Note-relational Database

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