# Cardinality - I really need help

Date: Sat, 04 Aug 2007 12:23:28 -0700

Message-ID: <1186255408.948471.113990_at_d55g2000hsg.googlegroups.com>

hello

I would really need some help here

*>>1)
**>>The following quote ( well I shortened it a bit ) is from a chapter
**>>briefly describing MARTIN E-R notation:
**>>"Say we have entities ORDER and PRODUCT. One ORDER >>must include at least one product, but it can also have more
**>>than one product. One PRODUCT can be related to zero or
**>>more ORDERS. Thus cardinality of PRODUCT is ( 1, N ) and
**>>cardinality of ORDER is ( 0, N )" But to my understanding, the
**>>cardinality of ORDER entity should be ( 1,N ) --> where 1
**>>means min number of connections and N max number of
**>>connections an individual ORDER entity can have. And
*

>>cardinality of PRODUCT entity should be ( 0,N ). But my book claims just the opposite!

*>>
*

> I would have said it differently: the cardinality of the > PRODUCT-ORDER relationship with respect to ORDERS is > (1,N). the cardinality of the PRODUCT-ORDER relationship > with respect to PRODUCTS is (0,N). I think you and I are on > the same page, but may need to reread Martin.

a)

Here is a picture from my book:

http://s195.photobucket.com/albums/z194/beginner16/?action=view¤t=kardinalnost.jpg

Book claims symbols on the graph show cardinality of E2 --> E2 = (0, 1). Huh?! Symbols on a graph show that E1 instance can be connected to zero or one instance of E2, so shouldn't cardinality of E1 be (0,1)?

b)

We have two entity types A and B:

A1 is an entity instance of type A

B1 is an entity instance of type B

As far as I understand cardinality, if we have two entity types A and B, and if A1 can be connected to one or more instances of type B, while B1 can only be connected to zero or one instances of A entity type, then relationship is 0:N, as depicted in a picture:

Thus to my understanding:

* Cardinality of A1 = (1:N), where 1 tells minimum number of
connections ( of some type ) that A1 must have, while N tells max
number of connections.

- Cardinality of B1 = (0, 1)

But the following suggests differently:

> I would have said it differently: the cardinality of the > PRODUCT-ORDER relationship with respect to ORDERS is > (1,N). the cardinality of the PRODUCT-ORDER relationship > with respect to PRODUCTS is (0,N). I think you and I are on > the same page, but may need to reread Martin.

If I'd go by the quote above, then cardinality of A1 = (0, 1) and of B1 = (1, N), which doesn't make sense!

thank you Received on Sat Aug 04 2007 - 21:23:28 CEST