ER diagram: product and components

The diagram contains two entities, a many-to-many relationship, and a recursive relationship on Component.

Entities and attributes

• Product (p_no, desc, cost).
• Component (c_no, desc, measure).

Relationships

1. comprised_of between Product and Component: M : N. A product is made of many components, and one component can be used in many products.
2. make on Component (recursive, unary): a component is made from other components. The relationship has the attribute no (how many units of the sub-component are used). Roles: assembly and part.

ER diagram

 (p_no)(desc)(cost)
        \  |  /
       [ Product ]
           |
           M
      <comprised_of>
           |
           N
       [Component]----(c_no)
        |  |   |  \
        |  |   |   (desc)(measure)
        |  |   +---------+
        |  |             |
        1  +----<make>---+  (no)
      (assembly)  N     (part)

The recursive line leaves Component and returns to it, labelled with the roles. In the relational schema this becomes: Product(p_no, desc, cost), Component(c_no, desc, measure), Comprised_of(p_no, c_no), Make(assembly_c_no, part_c_no, no).