Making Data Models Readable - High-Performance Web Databases: Design, Development, and Deployment - page 241

Exhibit 16-2. Chen's relationships.

and issues which go into database design are by and large not of interest to
the general public.

At its simplest, a relationship is represented in most tech-
niques by a simple line connecting two entities. Optionality and cardinality
are then shown as characteristics of the relationship by additional symbols.
In those cases where a relationship itself has attributes and other relation-
ships to it, many techniques endorse simply defining an entity to represent
the relationship as itself being a thing of significance to the enterprise. A line
item, for example, is essentially a relationship between an order and a prod-
uct, but most approaches treat it as an entity in its own right.

Relationships.

Chen, on the other hand, adds a different symbol (a rhombus) in the
middle of the relationship line, to allow him to show these entity-like char-
acteristics, without losing the fact that it is something other than an entity
that is being represented.

Exhibit 2, for example, shows the three-way relationship - - ,

10

PROJ PART SUPP

which is the fact that a is supplied to a by a . An alter-

PART PROJECT SUPPLIER

native to this approach would be to replace with an entity of

the same name. (Better yet, replace it with an entity that has a meaningful
name, such as

- -

PROJ PART SUPP

. See the discussion of names following.) This would
mean the same thing, while eliminating the second symbol type. Chen
would probably argue that this second approach camouflages the object's
role as a relationship. The question to answer is: how important is that to

SUPPLY