Re: One-To-One Relationships

On 1 dec, 15:03, JOG <j..._at_cs.nott.ac.uk> wrote:
>
>
>
> [...] let me give an example as to why I find
> the breakdown into entities and relationships deleterious. Say I have
> two entity types staff_members and subjects, and a relationship
> teaches:
>
> staff_member -- teaches --> subject
>
> This is all good and fine until a requirement changes that we need to
> record the day the lecture is given on.

Yes. Relationships may become Entities, Roles may become Relationships, Attributes may becomes Relationships, etc. As David remarked, there are ER dialects like ORM that smoothen this a bit, but you can never make that really go away. These problems have counterparts in the RM where foreign keys may become tables by themselves, or tables are split because a one-to-one relationship has become a one-to-many one.

> To denote the is new
> information, well I now longer haver a binary relationship, but a
> ternary one, and that requires a rewrite of the E/R representation
> (given that it is a graph). I need a new lecture associative entity
> with 'day' as an attribute.

It seems you talking about a particular ER dialect here. In many, and certainly not in the classical one, there is no distinction between binary and ternary relationships.

• Jan Hidders