the relational model for database management systems cis 205, fall 2004 last edited september 17,...

The Relational Model for

Database Management Systems

CIS 205, Fall 2004

Last edited September 17, 2004By C. Herbert ©2004, all rights reserved

CIS 205 Relational Model p.2

Most modern database management systems use

the Relational Model, in which data is organized

as a set of tables.

Name Address Phone Number

Joe Smith 111 Oak Ave 555-1111

Mary Jones 222 Main St 555-1212

George Bush 1600 Penn. Ave 555-1000

John Kerry 12 Lexington Rd 555-1313

The Relational Model forDatabase Management Systems



Tables are organized into rows and columns.









Each row represents one “record” in the database.









Each column represents a different “field”.








There is a table for each type of item in a data base.

For example, SEPTA might have a table for drivers, a table to

keep track of buses, and another table for bus routes.






Route # Start End

23 3600 S 11th St 9000 Germantown

9 Andorra Plaza Broad and South

2 5400 N. 17th 3300 S. 16th

Bus # Make Model

2078 GM Starliner

2079 Mercedes Dymaxion

2080 GM Sunliner



In relational database management, another name

for a table is a relation. We also talk about

entities, attributes and relationships.

An entity is what each table is all about.

There is a table for each entity in a database.



There is a table for each entity in a database.

In our SEPTA example we have tables for drivers,

buses, and bus routes.






Route # Start End

23 3600 S 11th St 9000 Germantown

9 Andorra Plaza Broad and South

2 5400 N. 17th 3300 S. 16th

Bus # Make Model

2078 GM Starliner


2080 GM Sunliner



The word entity refers to the general object

for each table. Each specific object is called

an instance of the entity.

There is a row for each instance of the entity.








There is a row for each instance of the entity.








An attribute is something that provides information about, or describes, an entity.

There is a column for each attribute of the entity.



A relationship is formed when two tables share

an attribute. (have a common column)

Data in one table is “cross referenced” with data in

another table through the column they have in

common. (the shared attribute)

“Cross-referencing” data is one of the keys to

effective database management.



In our SEPTA example, bus routes can be cross

referenced with buses to find out what make and

model of bus is being used on each route.

Route # Start End Bus #

23 3600 S 11th St 9000 Germantown 2078

9 Andorra Plaza Broad and South 2080

Bus # Make Model

2078 GM Starliner


2080 GM Sunliner



In our SEPTA example, bus routes can be cross

referenced with buses to find out what make and

model of bus is being used on each route.




Bus # Make Model

2078 GM Starliner


2080 GM Sunliner

Here, we can see that a GM Starliner is scheduled for use

on Route 23. Is it too big for the streets on that route?



In order for this cross referencing to work, we must

be able to uniquely identify each instance of an

entity. In other words, each row in a table must

have a unique value for a particular column.

Bus # Make Model

2078 GM Starliner


2080 GM Sunliner

We call such a column the primary key for the table. The primary key uniquely identifies each instance of an entity.



Here the Bus # is different for each row in the

database. The Bus # can be used as the primary

key for the bus table.

Bus # Make Model

2078 GM Starliner


2080 GM Sunliner

We call such a column the primary key for the table. The primary key uniquely identifies each instance of an entity.



A column in one table that is the primary key for

another table is called a foreign key.




Bus # Make Model

2078 GM Starliner


2080 GM Sunliner



Here, Bus # is a foreign key in the bus route

table, because it is a primary key in the bus table.




Bus # Make Model

2078 GM Starliner


2080 GM Sunliner

Foreign keyPrimary key


Sometimes a single column is not enough to form

a key field.








In this table, the Name column by itself could be a

primary key, assuming no two names are the

same.








However, if the first name and last name are in

separate columns, then both must be used to form

a primary key.

First Name

Last Name

Address Phone Number







A key that is formed from more than one column is

called a composite key.


First Name

Last Name

Address Phone Number





the relational model for database management systems cis 205, fall 2004 last edited september 17,...

Documents

title page

model of bus

effective database management

bus routes

septa example

common column data

data base

track of buses