Re: grouping a many to many relation set

On Wed, Dec 01, 2004 at 06:57:54AM +0100, Johan Henselmans wrote:
> Richard Huxton wrote:
>
> > I think what's missing here is the explicit statement of which group
> > these belong in. Without a value to sort/group by, there's nothing for
> > your queries to "get a grip on".
> >
> > So - add a "group_id" column to the bank-book and receipt tables. Create
> > a sequence to generate group id's on demand.
>
> Thanks for the reply. Adding a group_id column would defeat the whole
> purpose of the relational model. I do not want to add a grouping
> beforehand.

How is an application going to know which records belong to which
groups without a group ID? Or is a group ID acceptable as long as
it's not part of the data, but rather generated by the query or
function that does the grouping?

> The grouping should take place according to certain criteria, in
> this case: group all the records that have at least one of two
> attributes in common.

What about chains like this:

bankbookdetid | receiptid
---------------+-----------
100 | 1
100 | 2
101 | 2
101 | 3
102 | 3
102 | 4

Should 1 be grouped with 2, 3, and 4 since 1 has an attribute in
common with 2, 2 has an attribute in common with 3, and 3 has an
attribute in common with 4? Or doesn't your model permit this
situation?

--
Michael Fuhr
http://www.fuhr.org/~mfuhr/