Re: circular relationships ok?

Volker Hetzer schrieb:
> Alexandr Savinov schrieb:

>> Volker Hetzer schrieb:
>>> Alexandr Savinov schrieb:
>>>> Volker Hetzer schrieb:
>>>>> Hi!
>>>>> Just in general, are circular relationships something that
>>>>> can always be avoided?
>>>>> Or, given a model with a circular relationship, possibly
>>>>> spanning several tables, is there a way to get rid of them?
>>>>
>>>> One approach consists in using the concept-oriented data model as 
>>>> follows.
>>>>
>>>> 1. Order your tables so that if table A references records from 
>>>> table B, then A is positioned under B. For example, if an order has 
>>>> a field with its customer then you position them as follows:
>>>>
>>>> Customre
>>>>   |
>>>> Order
>>> So, what if A references B, B references C and C references A?

There is not such thing as "a row reference a bunch of rows". Any reference is a single thing. A row can reference only one row. Multi-valued attributes are implemented indirectly. In diagram this means changing direction of arrows, i.e., any downward arrow means a one-to-many relationship or multi-valued attribute.

I assume that you mean the following (maybe I am wrong):

a row from A is referenced from a bunch of rows in B, a row from B is referenced from a bunch of rows in C, each row (c) in C references zero or one row a' in A

Then it is modeled as follows:

A
| \
B |
| /
C

Here for each a in A we have many b in B. For each b in B we have many c in C. In the opposite direction: each c in C references one b in B and one a in A. Each b in B references one a in A.

>> Now assume that you have a cycle. This means that an element is a 
>> member of itself and it is normally interpreted as a non-sense.

If it is a loop then my opinion is that they should be permitted. Formally, we can avoid them but the life will be too hard. In addition, handling them is not a difficult task - loops (self-references) have clear and unambiguous semantics. So if an employee needs to reference a manager which is also an employee then it is not a problem.

If you have cycle then it is worse. It shows that something is wrong. However, I cannot exclude that sometimes this hack can be unavoidable. We need to separate theory and practice.

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