Portland Access User Group

Portland Access User Group Conference

September 28-30, 2019

The PAUG Database Designer International conference brings together a wide range of Access developers, consultants, power users and Access enthusiasts. This marks the 21st anniversary of the conference. We will once again be returning to the peaceful and natural surroundings of the Conference Center at Silver Falls State Park, which lends itself to a climate that fosters learning, creativity, and socializing.

Tuesday, June 13, 2017

Entity-Relationship Diagramming: Part III

In this Entity-Relationship series, I'm attempting to present an alternative to standard, decomposition-style normalization.

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.

Figure 1: Relationships Between Entities.

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.


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