weird... about 1:! generalization and 1:m non identifying...
I think that these examples can make it better to understand some of
those terms... I am quoting from Database Systems -- Design,
Implementation Management fouth edition by Rob Coronel (page 23)
Conceptual Modules use three types of relationships to descrive
associates amond data: one-to-many, many-to-many, and one-to-one.
Database designers usually use shorthand notations 1:M, M:N, and 1:1
for them, respectfully. The following examples illustrate the
distinctions among the three.
1. *ONE-TO-MANY Relationships* A painter pains many diffrent
paintings, but each one of hem is painted by only that painter. Thus
the painter (the one) is related to the paintings (the many).
Therefore, database designers lable the relationship PAINTER paints
PAINTINGS as 1:M. Simillarly, a customer account (the one) might
contain many invoices, but those invoices (the many) are related to
only a singe customer account. The CUSTOMER generates INVOICE
relationship would also be labled 1:M
2 *MANY-TO-MANY Relationship* An employee might learn many job skills,
ans each job skill might be learned by many employees. Database
designers label the relationship EMPLOYEE learns SKILL as M:N.
Similarly, a student can take many courses, and each course can be
taken by many students, thus yielding the M:N relationship label for
the relationship for the relationship expressed by STUDENT takes
COURSE
3 *ONE-TO-ONE Relationship* A retail company's management structure
may require that eaco one of its stores be managed by a single
employee. In turn, each store manager -- who is an employee -- only
manages a single store. Therefore the relationship EMPLOYEE manages
STORE is labled 1:1
Hope that this helps... as per the non identifying and the
generalizations... DUNNO
On Thu, 30 Sep 2004 13:14:03 -0400, Joshua Beall [EMAIL PROTECTED] wrote:
Hi All,
I've been taking a look at DB Designer 4, and looking through the
documentation (http://www.fabforce.net/dbdesigner4/doc/index.html) I am a
little unclear on some of their nomenclature:
'1:1' - Ok, one to one. Got it.
'1:1' generalization - Don't know this. Obviously different somehow from
one to one, but how?
'1:n' - One to many, I assume.
'1:n non identifying' - Nonidentifying? What does this mean?
'n:m' - Many to many? Again, not sure.
Can anyone help clarify?
Thanks!
-Josh
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