In Entity-Relationship Diagramming: Part I, I grouped all the objects in my business narrative into either Entities or Attributes. Entities are groups of Attributes and Attribute describe Entities.
In Entity-Relationship Diagramming Part II, I defined the relationships between my entities. Relationships store information about how your Entities interact. Figure 1 shows where I left off.
Unfortunately, I'm not done yet, for two reasons: 1) as I said in What is Normalization: Part V, many-to-many relationships cannot be directly implemented in a relational database, and 2) I still have an unassigned attribute. So first I'll rationalize the many-to-many relationship and then take another look.
To rationalize a many-to-many relationship between two tables, you create a third table -- an "intersection" or "linking" table. Then you create one-to-many relationships between the linking table and each of the main tables, with the "many-side" of both relationships on the linking table.
As you can see above, Employee and Certifications have a many-to-many relationship, so I need to create a new entity (Employee/Certifications). Sometimes linking tables have logical names. Other times, they don't. In that case, I simply combine the names of the base tables.
Figure 2 shows how the rationalized relationship is diagramed.
Figure 2: Rationalized Employee-Certification Relationship
Now I can see where to put my unassigned Certification Date field. The Employee/Certification entity represents a certification for a particular employee and that can be given at only one time. Therefore the Certification Date field goes in this new entity. Figure 3 shows the completed Attribute Grid.
Figure 3: Final Attribute Grid
Now that I've got all the relationships between my entities identified and assigned all the attributes, I can put it all into one diagram.
Figure 4: Final E-R Diagram
Now I've got all the pieces. All that's left is to implement my diagram in Access (or some other database). In Entity-Relationship Diagramming: Part IV, I'll do just that.