+1  A: 

In some cases, it could mean the difference between a foreign key being allowed to be NULL or not.

In other cases it might be something you need to check via constraints or application code.

Unfortunately I'm unfamiliar with the notation used in the diagram you post so I can't tell you what it means there.

Lasse V. Karlsen  2009-05-29 09:08:01
+2  A: 

If the child of the relationship (in the example above set as "classified"), is optional then the column is defined with the database as allowing null records. i.e set to NULL. This means that you can either specify no records as being linked from classified to category, or many records.

Calanus  2009-05-29 09:11:27
"or many records" -> do we know from the dot that it permits a "many" relationship?
eggdrop  2009-05-29 09:20:54
no - it is from the "crows feet" on the line that we know it is a many relationship. Actually looking at the diagram shows that category to classified is a many to many relationship.
Calanus  2009-05-29 10:42:23

related questions

Is there a way to script diagrams in SQL 2000 (or save them another way)?
Distributed System Diagramming Best Practices?
Sprint Cumulative Flow Diagram
Who uses a digital notepad for saving UML and other diagrams?
WPF Diagramming Library
Good tool for drawing SCM code branch diagrams
Loading an existing database into WWW SQL Designer?
Decipher database schema
Compartment items not displayed in DSL diagram
Good tool to generate dependency diagram?
How do I reference a diagram in a DSL T4 template?
Recommended Eclipse plugins to generate UML from Java code
Which tool do you use to draw simple diagrams/pictures to illustrate a technical point?
Which tool do I use if I want to map a programming framework visually and see the relationship between all entities?
Is there a good WPF diagrammer / toolkit / provider ?
Diagramming Software for a Developer/Designer
Open 2 Visio diagrams in different windows
Database Schema Diagram Design Tool
Suitable E-R notation for introductory DB design course? Good/free diagram tools?
Is UML practical?
VS.NET Application Diagrams
Entity diagrams in ASP.NET MVC
What's the best UML diagramming tool?
What program can I use to generate diagrams of SQL view/table structure?
Auto Generate Database Diagram MySQL