Chapter 5

Relationships & Queries in Databases

Types of Relationships

One to One Examples? Analysis Technique

Consider ThingA and ThingB Can ThingA be related to more than one ThingB? Can ThingB be related to more than one ThingA? If the two answers are NO, then it is a one to one

relationship.

Types of Relationships

One to Many Examples? Analysis Technique

Consider ThingA and ThingB Can ThingA be related to more than one ThingB? Can ThingB be related to more than one ThingA? If the answers are no and yes, or yes and no, then

the relationship is one to many.

Types of Relationships

Many to Many Examples? Analysis Technique

Consider ThingA and ThingB Can ThingA be related to more than one ThingB? Can ThingB be related to more than one ThingA? If the answers are yes and yes, then you have a

many to many relationship

Hierarchical Leads to Serious Data Redundancy

Hierarchical

Network

NetworkDifficult to manage

and implement relationships

RelationalThe most popular

structure

Relational

CustomersCID FName LName Address

508 Eric Breimer ...

509 Andrew Zych ...

510 Greg Smith ...

ProductsPID Description Cost

199 Viagra $45.99

200 Tooth Paste $2.58

201 Hair Gel $5.99

OrdersOID CID PID Quantity

001 508 199 500,000

002 508 201 2

003 510 201 1

Find the name and hire date of the manager working on the sales manual project

Schemas

Schema - A description of the database

Subschema – describes a subset of the database and which users have access to this subset

Data Definition Language

Language Used to describe Schemas and Subschemas

Describes relationships between different data

Provides a Logical view of the data

Data Dictionary Entry

A more detailed description of the data in a database

Specifies data types and ranges

Assists programmers in understanding the data

Logical vs. Physical Access Paths

Database Management Systems concentrate on Physical access to the underlying tables Concurrency control Query’s Creating/deleting tables

MIS systems are often software packages that (Logically) interface with a DBMS monthly reports charts automated inquiries