E-R Model?

[Nocturnal]

Ok I have this book from a class I took at a community college on Access.
It has a few chapters that deal with the E-R model. I kind of get confused
when they start saying entity classes and then entity instances and then
attributes, etc.

Is entity the same thing as a table? And then an entitiy instance the same
thing as a row or field?

 

[Ken Snell \(MVP\)]

An entity can be a table, or a form, or a report, or a class module.
Essentially, it is a self-contained set of data/properties that describe
"one thing".

An entity instance is one "instance" of that entity. For a table, one
probably could say that the entity (the table) has just one instance --
itself. For other objects, e.g., a class module, an instance is one
instantiation of that object. In a database, you can have multiple copies of
a form open at one time, for example, each with its own unique set of data
records.

 

[Dirk Goldgar]

Ok I have this book from a class I took at a community college on
Access. It has a few chapters that deal with the E-R model. I kind
of get confused when they start saying entity classes and then entity
instances and then attributes, etc.

Is entity the same thing as a table? And then an entitiy instance
the same thing as a row or field?

In a relational database like Access/Jet, an entity-relationship model
translates pretty much as you said. An entity class is a represented as
a table, a single instance of that entity class is represented as a row
in that table, and the attributes of that entity are fields (or columns)
in that table.

 

[Nocturnal]

I love how authors really love to over complicate things. They could have
easily said this is analogous to this and this is analogous to this.
Instead they try to make it all complex as though it is an entirely
different beast. I hate authors that do that. C'mon!

 

[John Vinson]

Is entity the same thing as a table? And then an entitiy instance the same
thing as a row or field?

An Entity is a person, thing or event out in the real world. A Class
of Entities is a collection of entities of interest to your database
application - for example in the Northwind sample database, Customers
are one class of entities; Products are another; Suppliers are yet
another.

A Table is a computer representation of a class of entities; each row
in the table is a representation of one member of that class.

John W. Vinson[MVP]

 

[David F Cox]

Authors write for different readerships and for different purposes. English
is a very imprecise language, full of ambiguities. In order to discuss very
precise mathematical concepts they have to use a language that is not
commonly understood. Without this precise language it would not be possible
to make software that works as well as it does. There are authors that write
for those that build our tools, and others that explain it it "for dummies".
There are few authors that can span the divide, find those and treasure
them.