diagramerd
In an ERD what makes an entity "optional" vs "required"?
As I understand it, the white dot indicates "optional" while the black dot means "required".
If so, a Category is required while a Classified is optional.
But what does that actually mean in the database - to say that a Category is required and a Classified is optional?
Solution
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.
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