I know disjoint and overlapping, but what does the union like sign mean in an enhanced entity-relationship (EER) diagram...? For example: enter image description here In other words, what does this diagram mean?


2 Answers 2


It means that 'Books', 'Chapters' and 'Articles' are sub-classes of the super-class 'Publication'.

'Books', 'Chapters' and 'Articles' are categories of 'Publication'

You can check out this EER reference for more details and what not.

  • This answer is correct. Some people might be misled by the fact that the link uses the triangle to indicate the relationship. The link really is talking about the same thing. For a discussion of table design of sub-classes dba.stackexchange.com/questions/193394/… Dec 30, 2018 at 13:03

It means that the publication can be of type chapter, article, or book. That makes publication different from overlap and disjoint.

When it comes to disjoint or overlap of entities, the topic becomes about a super class and a subclass.

This, however, is more like a kind of category.

For example, if an organization has an owner, and if the owner can be a person, a company, or a bank, then it is a union of entities, if they are separate "tables".


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